Here's my system:
In addition to damage, when you crit you get to roll a special effect. A special effect can be anything from getting knocked back to something like decapitation. The important thing is, the crit reflects the severity of the blow instead of just being random extra bad news. In other words, you'll only get decapitated if the crit kills you anyway.
I break crits down into four severities:
Major Wounds are those that deal less than half the character's current (not full) hit points. (NB- this is to keep critical hits dangerous to high-level characters; otherwise a crit is "just another 30 hp.") All crits are at least major wounds, simply by virtue of being crits. Severity is 1d12.
Grievous Wounds deal over half the victim's current hp but still leave him above 0. Severity is 2d12.
Incapacitating Wounds are those which deal enough to reduce the victim to 0 or less hp, but leave him alive. Severity is 4d8.
Lethal Wounds are those which kill the victim. Severity is 3d6+22.
After rolling the crit's severity, consult the following chart:
1 Knocked Back 5’
2 Knocked Prone
3 Staggered 1 round
4 Dazed 1 round
5 Disarmed
6 Lose Shield (or off-hand weapon)
7 Break Finger, Toe or Nose; -1 to checks with hand/foot
8 Break Ribs (1d6); take 1d6 nonlethal damage whenever you run/charge
9 Lose 1d6 Teeth
10 Break Fingers (1d5) or Toes (1d5) or Lose an Ear
11 Break Hand, Foot, or Face; -2 to anything using injured part
12 Break Jaw; -2 to Bluff or Diplomacy, 20% verbal spell failure
13 Break Lower Arm, Lower Leg or Collarbone; -4 to anything using injured part, aggravating it deals 1d6 nonlethal damage
14 Damage to Armor, Shield or Weapon as well as target
15 Muscle Damage; take 1d4 Str damage
16 Internal Damage; take 1d4 Con damage
17 Break Upper Arm, Thigh or Sternum; -4 to anything using the injured part, aggravating it deals 1d6 lethal damage
18 Hamstrung, -10’ speed
19 Lose an Eye
20 Lose Fingers (1d5) or Toes (1d5) or Nose
21 Lose Hand or Foot
22 Deafened
23 Stunned 1d4 Rounds
24 Lose Lower Arm or Lower Leg
25 Lower Jaw Ripped Free; -8 to Diplomacy checks, 75% spell failure
26 Gutted; take 1d6 Str and Dex damage, 50% chance to tangle feet each square of movement, which inflicts 1d6 hp of damage to the victim
27 Vocal Cords Destroyed; no speech or verbal spellcasting
28 Brain Damage, 1d4 points each of Int, Wis and Cha drain
29 Knocked Unconscious 5d20 Minutes
30 Shattered Knee or Elbow; completely unusable, requires a regenerate to fix
31 Both Eyes Put Out, Blinded
32 Spine Shattered, Paralyzed (roll 1d6: 1– total; 2- neck down; 3- waist down; 4- left side; 5- right side; 6- arms only)
33 Throat Slit
34 Groin Destroyed
35 Heart Destroyed
36 Brain Destroyed
37 Decapitated
38 Unseemed from Nave to Crotch
39 Head Pulped
40 Cut in Half
Appropriate magic will fix such injuries. In the case of broken bones or the like, a single healing spell that cures damage equal to the damage taken in the crit will fix it.