There's a certain appeal in taking dumping an ability score you don't need in order to maximise another one. Particularly in the early game where having an extra +1 on AC, or to hit, or DCs and so on can really be the difference between being regularly effective and dead. Often, the actual downsides aren't played up enough though. At best, the main thing that happens is there's a monster that targets your dump stat and you're out of it for a while. So, the point of this post is to extrapolate from some actual rules to what is actually the case when you have a dump stat.
Strength
There are mostly proper rules for how bad strength is for you, in terms of carrying capacity. For a medium bipedal creature of ten strength or lower, your maximum load in pounds is 10 x your strength. A medium load is anything more than 1/3 of your maximum and a heavy load is 2/3 of your maximum. Medium load reduces your speed as armour does, restricts your maximum dexterity bonus to armour to +3 and gives a -3 to all physical actions. Heavy load decreases max dex to +1 and makes the penalty -6. You can lift up to double that, but you're very definitely staggering under the weight, losing your dexterity bonus to AC and can only move 5 feet per round as a full round action. You can push or drag up to 5 times your maximum load.
For a standard adventurer, minimal gear (the basic protective items, clothes and a light weapon) come to about 10 lbs.. This usually means something like this:
1 Strength (-5 modifier): A dagger and absolutely minimal clothes put you at medium load, reducing your speed and giving you a -3 to anything physical. Adding thing that are likely necessary like a Spell Component Pouch, spellbook or money also requires a belt and or backpack, putting you above maximum load. What about a handy haversack? You need your spell component pouch right there, and it weighs 5, so you're still over max load. Basically, you're always denied dexterity to AC, take -6 to all physical actions (including attack rolls &c. I think), and can move 5 feet only as a full round action.
3 Strength (-4 modifier): You can wear a proper set of clothes for adventuring, but if you do, that's it for your light load. You're stuck with peasant's garb like the 1 strength character more likely. You're just at medium load with the minimal gear of clothes, dagger, spell component pouch and handy haversack, so you're still slowed and stuff. Comparatively necessary things like cloaks of resistance, headbands, will also weigh you down quite quickly, and unless you're explicitly changing your money into lightweight forms you'll need multiple haversacks or a bag of holding... very easily puts you at heavy load, if not maximum. You basically always have medium load though, even with magic, and will occassionally have heavy load.
5 Strength (-3 modifier): Minimum normally possible with point buy including race penalties. You can have minimal basic things on you at all times, without being encumbered, just. Even without magical storage. This is things like clothes, light wearable magic items, a spell component pouch, a light weapon and maybe a backpack or something similar. A bag of holding type I definitely puts you into medium load, and better versions put you into heavy. You will probably have medium load unless you're very careful, but it's workable.
7 Strength (-2 modifier): Minimum normal with point buy. You don't get a medium load until over 23 pounds now, so you can actually have clothes, other light necessities and a handy haversack and be fine, if you don't have much else, but pulling out something heavy like a metamagic rod will put you over. If you have other people to carry heavy things like piles of money. If you're by yourself, you either have very little on you (unless you have magical storage) or you're at medium encumbrance.
8 Strength (-1 modifier): Average for weak races. You're usually fine, and can actually afford to wear lighter forms of armour without penalty. Leather (weighing 15 lbs.) at the most. You can skimp on armour and get a bag of holding type 1 and or handy haversack instead with no penalty, but both will put you over. Not into heavy, but definitely medium.
10 Strength (+0 modifier): Normal average, supposedly. With a 34lb. start for a medium load, once you have magical storage you'll probably never need to worry about this again. You can wear your basics, have some weapons, and pretty much wear armour of any type with only normal armour penalties. Studded Leather and Chain shirts are probably the only exception, since they're heavy enough to make an impact. If you've got a lot of stuff you're probably in medium encumbrance, but you're unlikely to reach heavy.
12 Strength (+1 modifier): Recommended minimum. Like 10 strength, you might need to worry if you've got lots of stuff or it's early game, but you won't need to worry about the better light armours. Basically, at this point and beyond, it's no longer a problem at any stage.
So, having gone from low strength to average, you can see how it changes from being barely able to move under the weight of your clothes to being okay if you have next to nothing to the problem not being an issue. The next obvious point is Constitution, since this indirectly has rules governing what it does.
Constitution
1 Constitution (-5 modifier): I'd assume there's a minimum 1 HP per HD or something. Otherwise, if you're this low, on average you lose HP when you level up, unless you're a barbarian. Expect to have single digit HP. Level 1 d6 HP characters will actually be knocked unconscious with a single point of non-lethal damage (minimum from any attack) or outright killed with 3 points of non-lethal damage. Probably killed by even the very weakest diseases with DCs ~12. Dead by middle age without magic.
5 Constitution (-3 modifier): Minimum possible with point buy for a weak race. 50:50 chance of getting positive HP on level up, even for a d6 character, and on average, you do actually get HP. That said, while positive, it's still guaranteed single digit. Very easily poisoned, disease prone, and likely to die without magical healing if injured, even for the fairly hardy classes.
8 Constitution (-1 modifier): Average for races with weak constitution. Fragile, but viable. They can't take many hits, but if you're far enough away from the front lines or have decent defence you'll be fine. By yourself you will probably die.
10 Constitution (+0 modifier): Normal average, supposedly. You're not taking any penalties at last, so it's fine. You can get by fine by yourself and take a hit or two. Acceptable for most practical purposes.
12 Constitution (+1 modifier): Minimum recommended. Most of the nastiest stuff is fortitude based, and you need those HP. At this point and beyond, mostly any problems you get will be easy to remedy.
And so, we move onto the other things to give an idea of what they should probably be like. I'll try to keep them on the same scale. That is, -5 is basically impossible to survive, -3 is very bad but fixable only to workable, -1 is workable but still bad, 0 is fine and +1 is minimum recommended.
Dexterity
1 Dexterity (-5 modifier): Nearly no manual control at all. Limbs move very rigidly, and fingers probably not at all. Impossible to hold things, except for those that are very large. Movement speed reduced to 5 feet as a full round action. Cannot stand up if fallen prone, and becoming prone is very easily due to heavily impaired sense of balance.
5 Dexterity (-3 modifier): Minimum possible for point buy for clumsy races. Limbs move very stiffly or constantly jitter. Movement speed and physical actions constantly affected as if carrying a medium load, probably, unless you spend special attention to what you're doing. Very prone to falling over and dropping things. 25% chance of falling prone, dropping held items or otherwise involuntarily moving in awkward situations.
8 Dexterity (-1 modifier): Average for clumsy races. Can move normally, but still slightly accident prone. 5% chance of tripping, dropping a held item or doing something awkward when jolted.
10 Dexterity (+0 modifier): Normal average, supposedly. No penalties or anything, and quite serviceable. You don't tend to do anything accidentally.
12 Dexterity (+1 modifier): Minimum recommended. Everything's mechanical upsides from here and there are no circumstantial penalties at all.
The mental stats are always harder, because they're sort of vague, comparatively. I usually feel like there's a lot of confusion with how they work, so I'm going with how I think they work, or at least how they ought to.
Intelligence
This is very much how much you can remember more than your ability to think. It does include the ability to think logically, since this is largely just following a formula.
1 Intelligence (-5 modifier): You have nearly no ability to remember things. Anything beyond 1 round ago you immediately forget and have no concept of object permanence.
3 Intelligence (-4 modifier): You can remember a few select things, if they're strong memories. A few words, if you've heard them a lot. You can usually remember things that happened in the last minute.
5 Intelligence (-3 modifier): Minimum possible for stupid races using point buy. You can speak simple sentences, with a vocabulary of up to 100 words. You can usually remember things that happened during the last few hours. You have a vague idea of some abstract concepts such as time, numbers and truth.
8 Intelligence (-1 modifier): Average for stupid races. Can speak coherently, if with unimaginative sentence structure and vocabulary. Can usually remember important events in their life. Remembers simple instructions, provided they're well explained or written down. Usually have a limited capability to think abstractly.
10 Intelligence (+0 modifier): Normal average, supposedly. If you're reading this, you're probably at least this level of intelligence, so just play as you would, because it's much harder to quantify intelligence with a score past here.
Wisdom
This, I consider to be more understanding and insight. It's all very well to know something, but understanding of why it is like it is is wisdom. Almost more like processing information, rather than just having it. To an extent, it's the creative ability, and the ability to think outside the box.
1 Wisdom (-5 modifier): Simply acts with no understanding of its actions, usually by whatever its instincts drive it to do. Mind control simply adds new desires, which the creature cannot recognise as being foreign.
5 Wisdom (-3 modifier): Almost always acts on impulse, but has an awareness of things like hunger, lust or desire. Almost childlike in nature. "Why did you eat that chocolate? It was poisoned!" "I was hungry!" or "Why did you touch the fire?" "It was pretty and I wanted it!"
8 Wisdom (-1 modifier): Average for foolish races. Usually have a reason for their actions, but don't think too hard about them and tend to dislike doing so. A very basic understanding of ethics.
10 Wisdom (+0 modifier): Normal average, supposedly. Like intelligence, past here is difficult to quantify and harder to roleplay without actually having that level of wisdom.
Charisma
It's defined as "a character’s personality, personal magnetism, ability to lead, and appearance". While it's easy to say it's just appearance with a low score, I'll primarily focus on the other bits. This one in particular shows how much people like you, so I'll also put in various reactions.
1 Charisma (-5 modifier): You have nearly no personality or will of your own whatsoever. If someone tells you to do something, you automatically do so, or if they even hint at it. While you can speak, you are always too shy or terrified of everything to ever step out of place or do something without someone telling you to. Other creatures view you as pathetic or mistake you for a broken slave. You are taken advantage of at every available opportunity.
5 Charisma (-3 modifier): You are terribly shy, fearful or anxious. If something asks you to do something dramatically opposed to your beliefs or you hate, you can speak out against it, but otherwise you usually do as they say. You stutter or pause a lot while speaking when gain the confidence to do so. You act on your own accord, but prefer to let someone direct you. Regularly paralysed by indecision and crushed by every rejection. Some people might stand up for you, but you are a definite bully magnet.
8 Charisma (-1 modifier): Average for shy creatures. Natural servants, but not automatically obedient. Prone to social anxieties or otherwise introverted because of not wanting to disturb other people. Have trouble making decisions and prone to giving up when difficulties arise. Slight stutters, nervous jitters and often slightly disliked.
10 Charisma (+0 modifier): See notes for Intelligence and Wisdom. Just normal human, and you can probably act as normal.
Strength
There are mostly proper rules for how bad strength is for you, in terms of carrying capacity. For a medium bipedal creature of ten strength or lower, your maximum load in pounds is 10 x your strength. A medium load is anything more than 1/3 of your maximum and a heavy load is 2/3 of your maximum. Medium load reduces your speed as armour does, restricts your maximum dexterity bonus to armour to +3 and gives a -3 to all physical actions. Heavy load decreases max dex to +1 and makes the penalty -6. You can lift up to double that, but you're very definitely staggering under the weight, losing your dexterity bonus to AC and can only move 5 feet per round as a full round action. You can push or drag up to 5 times your maximum load.
For a standard adventurer, minimal gear (the basic protective items, clothes and a light weapon) come to about 10 lbs.. This usually means something like this:
1 Strength (-5 modifier): A dagger and absolutely minimal clothes put you at medium load, reducing your speed and giving you a -3 to anything physical. Adding thing that are likely necessary like a Spell Component Pouch, spellbook or money also requires a belt and or backpack, putting you above maximum load. What about a handy haversack? You need your spell component pouch right there, and it weighs 5, so you're still over max load. Basically, you're always denied dexterity to AC, take -6 to all physical actions (including attack rolls &c. I think), and can move 5 feet only as a full round action.
3 Strength (-4 modifier): You can wear a proper set of clothes for adventuring, but if you do, that's it for your light load. You're stuck with peasant's garb like the 1 strength character more likely. You're just at medium load with the minimal gear of clothes, dagger, spell component pouch and handy haversack, so you're still slowed and stuff. Comparatively necessary things like cloaks of resistance, headbands, will also weigh you down quite quickly, and unless you're explicitly changing your money into lightweight forms you'll need multiple haversacks or a bag of holding... very easily puts you at heavy load, if not maximum. You basically always have medium load though, even with magic, and will occassionally have heavy load.
5 Strength (-3 modifier): Minimum normally possible with point buy including race penalties. You can have minimal basic things on you at all times, without being encumbered, just. Even without magical storage. This is things like clothes, light wearable magic items, a spell component pouch, a light weapon and maybe a backpack or something similar. A bag of holding type I definitely puts you into medium load, and better versions put you into heavy. You will probably have medium load unless you're very careful, but it's workable.
7 Strength (-2 modifier): Minimum normal with point buy. You don't get a medium load until over 23 pounds now, so you can actually have clothes, other light necessities and a handy haversack and be fine, if you don't have much else, but pulling out something heavy like a metamagic rod will put you over. If you have other people to carry heavy things like piles of money. If you're by yourself, you either have very little on you (unless you have magical storage) or you're at medium encumbrance.
8 Strength (-1 modifier): Average for weak races. You're usually fine, and can actually afford to wear lighter forms of armour without penalty. Leather (weighing 15 lbs.) at the most. You can skimp on armour and get a bag of holding type 1 and or handy haversack instead with no penalty, but both will put you over. Not into heavy, but definitely medium.
10 Strength (+0 modifier): Normal average, supposedly. With a 34lb. start for a medium load, once you have magical storage you'll probably never need to worry about this again. You can wear your basics, have some weapons, and pretty much wear armour of any type with only normal armour penalties. Studded Leather and Chain shirts are probably the only exception, since they're heavy enough to make an impact. If you've got a lot of stuff you're probably in medium encumbrance, but you're unlikely to reach heavy.
12 Strength (+1 modifier): Recommended minimum. Like 10 strength, you might need to worry if you've got lots of stuff or it's early game, but you won't need to worry about the better light armours. Basically, at this point and beyond, it's no longer a problem at any stage.
So, having gone from low strength to average, you can see how it changes from being barely able to move under the weight of your clothes to being okay if you have next to nothing to the problem not being an issue. The next obvious point is Constitution, since this indirectly has rules governing what it does.
Constitution
1 Constitution (-5 modifier): I'd assume there's a minimum 1 HP per HD or something. Otherwise, if you're this low, on average you lose HP when you level up, unless you're a barbarian. Expect to have single digit HP. Level 1 d6 HP characters will actually be knocked unconscious with a single point of non-lethal damage (minimum from any attack) or outright killed with 3 points of non-lethal damage. Probably killed by even the very weakest diseases with DCs ~12. Dead by middle age without magic.
5 Constitution (-3 modifier): Minimum possible with point buy for a weak race. 50:50 chance of getting positive HP on level up, even for a d6 character, and on average, you do actually get HP. That said, while positive, it's still guaranteed single digit. Very easily poisoned, disease prone, and likely to die without magical healing if injured, even for the fairly hardy classes.
8 Constitution (-1 modifier): Average for races with weak constitution. Fragile, but viable. They can't take many hits, but if you're far enough away from the front lines or have decent defence you'll be fine. By yourself you will probably die.
10 Constitution (+0 modifier): Normal average, supposedly. You're not taking any penalties at last, so it's fine. You can get by fine by yourself and take a hit or two. Acceptable for most practical purposes.
12 Constitution (+1 modifier): Minimum recommended. Most of the nastiest stuff is fortitude based, and you need those HP. At this point and beyond, mostly any problems you get will be easy to remedy.
And so, we move onto the other things to give an idea of what they should probably be like. I'll try to keep them on the same scale. That is, -5 is basically impossible to survive, -3 is very bad but fixable only to workable, -1 is workable but still bad, 0 is fine and +1 is minimum recommended.
Dexterity
1 Dexterity (-5 modifier): Nearly no manual control at all. Limbs move very rigidly, and fingers probably not at all. Impossible to hold things, except for those that are very large. Movement speed reduced to 5 feet as a full round action. Cannot stand up if fallen prone, and becoming prone is very easily due to heavily impaired sense of balance.
5 Dexterity (-3 modifier): Minimum possible for point buy for clumsy races. Limbs move very stiffly or constantly jitter. Movement speed and physical actions constantly affected as if carrying a medium load, probably, unless you spend special attention to what you're doing. Very prone to falling over and dropping things. 25% chance of falling prone, dropping held items or otherwise involuntarily moving in awkward situations.
8 Dexterity (-1 modifier): Average for clumsy races. Can move normally, but still slightly accident prone. 5% chance of tripping, dropping a held item or doing something awkward when jolted.
10 Dexterity (+0 modifier): Normal average, supposedly. No penalties or anything, and quite serviceable. You don't tend to do anything accidentally.
12 Dexterity (+1 modifier): Minimum recommended. Everything's mechanical upsides from here and there are no circumstantial penalties at all.
The mental stats are always harder, because they're sort of vague, comparatively. I usually feel like there's a lot of confusion with how they work, so I'm going with how I think they work, or at least how they ought to.
Intelligence
This is very much how much you can remember more than your ability to think. It does include the ability to think logically, since this is largely just following a formula.
1 Intelligence (-5 modifier): You have nearly no ability to remember things. Anything beyond 1 round ago you immediately forget and have no concept of object permanence.
3 Intelligence (-4 modifier): You can remember a few select things, if they're strong memories. A few words, if you've heard them a lot. You can usually remember things that happened in the last minute.
5 Intelligence (-3 modifier): Minimum possible for stupid races using point buy. You can speak simple sentences, with a vocabulary of up to 100 words. You can usually remember things that happened during the last few hours. You have a vague idea of some abstract concepts such as time, numbers and truth.
8 Intelligence (-1 modifier): Average for stupid races. Can speak coherently, if with unimaginative sentence structure and vocabulary. Can usually remember important events in their life. Remembers simple instructions, provided they're well explained or written down. Usually have a limited capability to think abstractly.
10 Intelligence (+0 modifier): Normal average, supposedly. If you're reading this, you're probably at least this level of intelligence, so just play as you would, because it's much harder to quantify intelligence with a score past here.
Wisdom
This, I consider to be more understanding and insight. It's all very well to know something, but understanding of why it is like it is is wisdom. Almost more like processing information, rather than just having it. To an extent, it's the creative ability, and the ability to think outside the box.
1 Wisdom (-5 modifier): Simply acts with no understanding of its actions, usually by whatever its instincts drive it to do. Mind control simply adds new desires, which the creature cannot recognise as being foreign.
5 Wisdom (-3 modifier): Almost always acts on impulse, but has an awareness of things like hunger, lust or desire. Almost childlike in nature. "Why did you eat that chocolate? It was poisoned!" "I was hungry!" or "Why did you touch the fire?" "It was pretty and I wanted it!"
8 Wisdom (-1 modifier): Average for foolish races. Usually have a reason for their actions, but don't think too hard about them and tend to dislike doing so. A very basic understanding of ethics.
10 Wisdom (+0 modifier): Normal average, supposedly. Like intelligence, past here is difficult to quantify and harder to roleplay without actually having that level of wisdom.
Charisma
It's defined as "a character’s personality, personal magnetism, ability to lead, and appearance". While it's easy to say it's just appearance with a low score, I'll primarily focus on the other bits. This one in particular shows how much people like you, so I'll also put in various reactions.
1 Charisma (-5 modifier): You have nearly no personality or will of your own whatsoever. If someone tells you to do something, you automatically do so, or if they even hint at it. While you can speak, you are always too shy or terrified of everything to ever step out of place or do something without someone telling you to. Other creatures view you as pathetic or mistake you for a broken slave. You are taken advantage of at every available opportunity.
5 Charisma (-3 modifier): You are terribly shy, fearful or anxious. If something asks you to do something dramatically opposed to your beliefs or you hate, you can speak out against it, but otherwise you usually do as they say. You stutter or pause a lot while speaking when gain the confidence to do so. You act on your own accord, but prefer to let someone direct you. Regularly paralysed by indecision and crushed by every rejection. Some people might stand up for you, but you are a definite bully magnet.
8 Charisma (-1 modifier): Average for shy creatures. Natural servants, but not automatically obedient. Prone to social anxieties or otherwise introverted because of not wanting to disturb other people. Have trouble making decisions and prone to giving up when difficulties arise. Slight stutters, nervous jitters and often slightly disliked.
10 Charisma (+0 modifier): See notes for Intelligence and Wisdom. Just normal human, and you can probably act as normal.
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