- They often favor the enemy. In many encounters, the party is outnumbered by weaker creatures, like kobolds or stirges. This gives the enemy a greater chance of scoring crits.
- It just adds more rules to the game. In a game with an abstract combat system, critical hits are redundant. It is already assumed that the standard damage roll represents the effectiveness of a successful hit. Critical hits, called shots, and other mechanics are all part of a sliding scale toward a combat system that is unneccesarily cumbersome.
- They are too frequent. I already have a problem with using d20 instead of 3d6 for attack rolls, because of its lack of a bell curve. Add the fact that some weapons in 3.5 allow a critical threat with 19 or 18 and I get pretty uncomfortable with the odds that crits will happen. A crit should be a holy crap moment, like when Robin Hood pierced an opponent's arrow, or when Perseus succeeded in beheading Medusa, with limited visibility, in one swing. They should be the RPG equivalent of a grand slam, a four-point play, or a runner-runner inside straight.
LYLE'S CRITICAL HIT AND FUMBLE SYSTEM
Rule Number One: Natural 20 does not automatically hit, and natural 1 does not automatically miss.
If defender is already at 50% max hp or less...
1. Threat on natural 20 (only if 20 hits).
2. Player re-rolls 3d6 for 2x damage (two damage rolls, not a multiplier).
3. If re-roll is natural 18, DM re-rolls for chance to use effects table. A hit scores.
1 Attacker breaks bone (failure to describe target negates).
2 If head wound, blood from eye socket blinds defender for 1d4 rounds.
3 Defender is stunned during next round.
4 If head attack, defender suffers trauma. Con save vs. knockout for 1d6 turns, 1d4-1 Int. points lost.
5 If head attack with piercing or slashing weapon, loss of eye - results in stun for 1d3 rounds, -4 circumstance penalty for subsequent actions during combat.
6 If flame attack, subject catches fire. Suffer fire damage until extinguished.
7 At DM's discretion, a companion is distracted by the carnage for one round.
8 Defender loses balance. Dex save to avoid falling on back and starting next round prone.
9 Defender recoils blindly, stumbling into whatever is within 5' behind him.
10 Defender crippled in specific spot (must be called); on subsequent rounds, the attacker may exploit the area with called shots, by trading up to 4 to penalty points for an equal amount of extra damage.
11 Defender involuntarily drops weapon.
12 If slash attack, defender loses appendage; called shot required. DM's discretion for specific penalty.
13 Massive blood loss results in Con check vs. unconsciousness every round (Dex check +4 for attacker to avoid slipping on blood).
14 If head shot called, vision is affected. Defender fights as if attacker is behind 50% cover until he gets a one-round break.
15 Defender's weapon is broken. Swords unlikely, wooden handles etc. better.
16 A possession other than the defender's weapon, like arrows in a quiver or a potion vial, is broken.
17 If slashing, an article of clothing is rended. Defender suffers -2 to hit penalty due to entanglement.
18 Something besides the defender's weapon - like a backpack, belt, or helmet - falls to the floor. Dex checks every round to avoid tripping, plus possible breakage, AC penalties and other effects.
19 Bladed weapon buried so deep, retrieving causes another damage roll, halved.
20 Attacker succeeds quickly enough to be granted a second attack at -2 (no crit chance).
If defender is above 50% max hp...
1. Threat on natural 1 (only if 1 misses).
2. DM rolls 3d6 privately. If the result is 3, DM rolls secret to hit for player. A miss results in a fumble.
1. Attacker drops weapon.
2. Attacker hits self for 1/3 damage, 1/3 subdual damage.
3. Attacker must succeed in a Dex check or lose footing, and be flat-footed for one round.
4. If attacker wears helmet, the helmet shifts, resulting in partial blindness for one round.
5. Attack overcommits and is off-balance, with flank or back exposed. Defender gets an Attack of Opportunity.
6. If near a post, soft ground, clay, trees, or anything else that can be pierced, a piercing weapon gets stuck. It takes a move action to remove the weapon.
7. The attack is wild; Those in adjacent squares must succeed in a Dex check or be hit for 1/3 damage.
8. Weapon clashes against armor, shield or environmental object, causing it to break.
9. If ranged attack, those in the rear of the defender must make a Dex save versus 1/4 damage.
10. This round is exceptionally tiring. Attacker is temporarily out of breath, and his limbs tremble from the repeated impact. 1d2 subdual damage, plus minus one Constitution point until the end of the battle.
11. Attacker gets tangled in his backpack straps, overcoat, or other loosely worn gear. A move action will cut the offending material free, or the attacker must succeed in a Dex check every subsequent round to avoid a minus one penalty on hit and damage rolls.
12-13. Blade dulls. Attack rolls reduced by one until sharpened. This effect is cumulative.
14. Blade loosens from hilt. Attack rolls reduce by one until secured or replaced. Not cumulative.
15. If the attacker used a ranged weapon, the weapon is broken beyond repair.
16. The attacker's fingers get pinched. 1d3 subdual, Dex check versus dropping weapon, with circumstance penalty of -4.
17. Attacker hits nearby environmental object for full damage.
18. Attacker gets sweat in eye. Blinded for one round. Dabbing brow is a partial action.
19. If the attacker is holding a weapon or spiked armor in his off-hand, he hits himself for 1 point damage.
20. The sound of combat alerts a wandering monster, local habitat permitting.
This guy also has some choice words on the subject. Check out his site.