The mechanical effects of those score can vary from negligible to disastrous. If you're stringent with encumbrance, the thief's 8 in strength may come up, or the low charisma cleric is stuck with a few less hirelings. In many cases almost forgettable. A low Constitution for a fighter is annoying, but playable, but even a -1 one to rolled hit dice is crippling to a Magic-user - you're averaging less than 9 total hit points at 5th level. The odds of survival are realistic for a susceptible person crawling around sewers and fighting monsters, but almost not worth trying. Making it to 2nd level on 1 hit point can be scary and exciting for those sessions. Still dreading a single hit from a halberd months later is tiresome.
We can do this better. Penalties to fundamental game activities can turn a quirk into frustration. Not only do flaws make the character more interesting, they should come up in sessions.
1. Remove or change most class minimums. The fighter with a strength of 8 is awesome. Maybe for the rarer classes (Paladin, Assassin) you compensate for their not meeting the minimums in some other way (The Paladin Rules), but usually the low score or lack of bonuses is painful enough for the standard classes.
2. Change how penalties from low attribute scores work. Use the disadvantage rules from 5th Edition (still may be too in some cases). Find effects that don't require tweaking every die roll - a poor constitution manifests in requiring more rest at night (can't take a watch shift) or twice as much rations due to gastrointestinal issues.
A few ideas:
- Strength - reroll door opening attempts, decreased encumbrance capacity, needs assistance in manual tasks (piling crates to block the door)
- Intelligence - ability to read/write, becomes a fan of the New Orleans Saints, inability to learn other languages, may misunderstand written or verbal communications (misinterpret NPC saying: "I don't know if it's poisonous" vs "it's not poisonous") or miscount coins
- Wisdom - saving through penalties are probably sufficient, becomes lost more frequently, poor perception (distance vision or other senses)
- Dexterity - clumsy or unlucky in catching or grabbing items (You are Jack Burton), misfortunes that hit once random party member (rock falls from ceiling) tend to hit you.
- Constitution - I think the examples above, plus the AD&D system shock and resurrection survival percentages are good. A simplified disease system would make this really shine.
- Charisma - loyalty percentage per AD&D, increased possibility of hirelings lying/stealing, noticeable feature (weird eyes, scar or tattoo), suspicious feature (extra finger, forked tongue, webbed fingers), poor reaction rolls in more mundane situations ("The innkeeper dislikes you and you end up sleeping in the stables").