The low end

Contrary to popular opinion, I don't consider a character with one or more low ability scores - even very low ones in the 3-5 range - to be necessarily unplayable or hopeless.  As I noted in my previous post, most characters generated with the 3d6 in order method - the ONLY method described in the B/X rules - are going to have at least one low score.  The ability score tables don't include modifiers for scores under 9 simply to fill space on the page.  They're meant to be used by player characters.  The game was designed to support play with characters who have weaknesses, and it's really a shame that so many players and DMs are so averse to the low end of the ability score spectrum as to effectively house-rule it out of existence.

Even though I accept this on an intellectual level, when the dice hit the table during character creation, I tend to go soft, and allow my players to re-roll characters with several "bad" scores.  This post is at least as much for myself as for my readers, to fortify my resolve that subpar PCs don't necessarily make the game any less enjoyable, and that I'm not ruining the game for my young players by insisting that they play the hand they're dealt, so to speak, rather than keep rolling until they get a character with more robust stats.

A -1 penalty isn't really all that consequential in most cases.  Yes, the guy with 7 Strength is a lot less effective in melee than the guy with a 17, but he's only slightly less capable than the one with a 12.  Modifiers of -2 and -3 are more serious handicaps, but only 4.6% of scores rolled on 3d6 warrant such penalties.  Less than half a percent end up with -3.  Even these scores, though, are not inherently unplayable.  It's only our expectations that make them seem so.  Clever players find ways to minimize the effects of low scores, either by avoiding actions where they would be a liability, or making themselves useful in non-stat-related ways.

Here's a look at the average dice rolls of the usual assortment of D&D dice with modifiers of 0, -1, -2, and -3, keeping in mind that no roll is adjusted below 1.

                                                                       0         -1        -2        -3      
                                                             d4     2.5     1.75     1.25     1.0
                                                             d6     3.5     2.67     2.0       1.5
                                                             d8     4.5     3.625   2.875   2.25
                                                             d10   5.5     4.6       3.8       3.1
                                                             d20  10.5    9.55     8.65     7.8
Chance to roll a 14 or better on d20:              35%   30%     25%     20%
(Typical saving throw)

With these figures in mind, let's look a little more closely at how being below average in different abilities affects a character in play.

Strength affects both attack and damage rolls in melee combat.  A character with a low Strength is probably well advised to avoid melee if possible, but isn't necessarily hopeless at it.  It just takes him a little longer to dispatch any given opponent.  At a -1 penalty, a character wielding a 1d8 weapon is about as effective as one of average strength with a 1d6 weapon, and only 5% less likely to hit.  At -2 and -3, the penalty to damage becomes a fairly significant hindrance.  Even so, a character with a decent AC and hit points might well make himself useful in melee by taking some of the pressure off the primary fighters.  He provides another target for monster attacks so the heavy hitters aren't absorbing all the attacks, and a few points of damage here and there really can make a difference. 

Of course, the best advice is often to avoid combat whenever possible, and this especially applies to parties with more than one physically weak character.

Intelligence affects literacy and languages known.  More so than most other abilities, the deficiencies of one character are readily offset by having one or two smarter members in the group.  Obviously a character with a low Intelligence isn't going to be reading the villain's journal or the inscriptions on tombs, but as long as there's somebody in the party who can read, even a total halfwit isn't much of a hindrance to a successful adventure.  Knowing a good range of languages is certainly a benefit to a party, but a couple PCs of above-average intellect or an elf or dwarf can easily cover the shortcomings of a less-than-bright comrade.

Wisdom affects saving throws vs. magical effects.  Saving throws come up a lot less often than attack and damage rolls, but the effects of a failed save are generally a lot more consequential than a missed attack or low damage roll.  Even if you expand the scope of Wisdom to affect all saves rather than just magic-based ones, though, the unwise character is only a little more likely to end up charmed, petrified, or poisoned than his companions of average prudence.  The best defense is smart play, to avoid having to roll a saving throw in the first place.  (I heartily subscribe to the philosophy that a saving throw represents a last chance for a character whose player has failed to heed signs of danger and done something which by all rights should kill the character.)

Dexterity affects Armor Class and attack rolls with missile weapons.  A PC with a low Dexterity isn't going to be the best archer, but it still may be worthwhile to carry a bow, sling, or a bandolier of daggers.  Firing at a charging beast with a reduced chance to hit is still more likely to be of help than standing there watching others fire at it.  Armor Class is a bigger deal, but the penalties still represent only a 5, 10, or 15 percentage point increase in the odds of being hit - significant, but hardly crippling.  A character with decent hit points still has good survivability even with a low Dexterity, and need not shrink from a fight for that reason only.

Constitution affects hit points per Hit Die, and so directly influences the survivability of every character.  Still, a fighter with a -1 penalty has about the same hp her die as a cleric, hardly a pushover.  A cleric with a -1 is as tough as a typical thief, and with better armor.  For magic-users and thieves, a low Constitution is a great hazard, especially at the -2 and -3 levels, when a d4 yields a maximum of 1 or 2 hp per level.  Low-Con fighter-types can still be effective in brief skirmishes.  Magic-users, thieves, and clerics with very low Constitution have an even stronger incentive than usual to avoid the perils of melee combat, and to tread carefully around places that may be trapped.  Such frailness demands greater caution in adventuring, but need not keep a character from the adventurer's life.  They must simply cultivate the virtues of caution, avoidance, stealth, and diplomacy to a greater degree.

Charisma affects reaction rolls and retainer morale.  Like Intelligence, having a low-Charisma character or two in the party is generally little hindrance.  Most commonly only one or two characters are the primary negotiators in encounters, so the high scores can be brought to bear while the low ones hang back to watch and whisper advice as needed.  Likewise, the high-Charisma PCs will probably hire the retainers for the whole party, though even the low-Charisma ones might be able to find a henchman or two and ensure their loyalty with greater shares of the loot.  Despite the undeniable benefits of having one or two charismatic PCs in the party, there is a reason why Charisma is the quintessential "dump stat" in games that allow rearranging of scores.

The bottom line is that low ability scores are really only crippling to the degree that players lack skill and imagination.  Skillful play can significantly reduce the amount of dice-rolling in a game, and thus limit the occasions when negative modifiers are a factor at all.  Judiciousness in picking fights and taking risks is an unmitigated virtue in D&D whether you're playing with 16s or 6s.  Even a "hopeless" character with several low scores and no high ones is a potentially successful adventurer, if the player gives it a shot.  Come to think of it, a whole group of them might actually be a fun change of pace from the typical badass adventuring party.