- You can’t do one-shot kills—when a foe has more than 1 HD, you usually need multiple hits to bring him down, and
- You can’t simulate performance degradation—meaning you’re just as effective at 1hp as you are at 50hp
We'll start by keeping hit dice and hit points. When an attack hits, roll damage normally and subtract from the target's hit point total. However, when an attack is a critical hit (see below) or when a combatant reaches zero hit points, the defender rolls on the Hit Location table to see what happens. 
Critical HitsA critical hit occurs when an unmodified attack roll result falls within the attacker’s critical range, which is 20 minus his class-based to-hit bonus. For example, a 4th-level fighter with a to-hit bonus of +2 has a critical range of 18-20. Thus, if his d20 attack roll shows a natural 18, 19, or 20, and the attack succeeds, it’s a critical hit. A monster's critical range is calculated the same way (though tops out at a critical range of 5-20).
Hit LocationsWhen a critical hit is rolled or a target is reduced to zero hit points,  roll on the Hit Location table (use a lame 1d6 or use these super-bitchin' dice):
1d6 LOC. MIN. DMG (1) NORMAL DMG (2) MAX. DMG (3) 1 Lf Arm Armour (4) STR -1 Lost limb (5) 2 Rt Arm Weapon (6) DEX -1 Lost limb (5) 3 Lf Leg Knockback 10' -1 to Move Lost limb (5) 4 Rt Leg -1 to AC -20# to Carry Lost limb (5) 5 Torso Knock down (7) Stunned (8) Shot to the Heart (9) 6 Head (10) Stunned (8) Knocked out Head shot (11) --------------- 1. Use if the damage roll is a natural "1." Penalties last until treated (i.e., 2-7 hp worth of healing). 2. Use if the damage roll is between "1" and the maximum die value. Save or effects are permanent. 3. Use if the damage roll is the maximum die value. 4. Reduce armour (or shield) protection by -1 (destroyed if reduced to zero). 5. Save to convert into a bleeder (death from blood loss in 1 turn/level unless given 4-14 hp worth of healing); if save fails, limb is gone, but no further damage. 6. Reduce weapon's damage die by one step (e.g., 1d6 becomes 1d4); weapon broken less than 1d4 damage. 7. Requires a full action to get back on your feet. 8. Stunned combatants are -1 to all rolls, AC, and Move. 9. Death in 1 round per level or HD unless given 4-14 hp worth of healing. 10. If wearing a helmet, save to treat as next lowest crit category. 11. Decapitation, throat slit, or weapon embedded in skull type of thing. Unless the victim was caught by surprise or somehow unable to act, he gets one immediate retaliatory strikeOption: Instead of modifying the to-hit or damage rolls, reflect weapon proficiency by modifying the hit location roll. More on this later, but you're now on notice that it's coming.
at -4before falling in a crumpled heap. _______________
Using this table, you'll quickly discover that it's hard to kill things, though combatants do get messed up. As a result, reserve this table for PCs, Big Bads, and "Boss" NPCs. For fodder, henchmen, and minions, just consider them gone when they reach zero hit points.
Soooo... the table. Too much? I like the idea of different results based the amount of damage rolled, but I think the entries can stand some improvement. That said, feel free to suggest said improvements in the comments section.
UPDATE (2/25/12): Edits resulting from playtesting: now all Max Damage crits are potentially life-threatening. Other results got shifted to the left, and I incorporated saves to make things more
Next Up, Unfocused thoughts on religion, Evan-style
Listening To: Tears for Fears, Sowing the Seeds of Love
- Based on the optional damage location system for Chimera RPG: http://www.welshpiper.com/hit-locations/; the zero hit point limit was inspired by Robert Fisher (http://web.fisher.cx/robert/infogami/Classic_D&D_injury_table).
- Traps, spells, falling, and other sources of damage won't inflict a critical hit, but they can reduce a target to zero hit points, so the system applies to anything that inflicts damage. This also allows you to deduct hit points for intangibles such as fatigue, disease,substance abuse, prolonged exposure to the elements, and going off on adventures without twinkies and beef jerky.
- A character at zero hit points is still active and able to fight. However, he does so at whatever penalty is imposed by the Hit Location table, and if he's hit again, he has to make another Hit Location roll.