But then we have another problem - in classic D&D, when you hit 0 you're dead. With players starting with single digit hit points, and often remaining in the teens for several levels, the game is extraordinarily lethal. This isn't the worst thing ever, but it does discourage players from bothering to invest in those characters until much later level, and that's a shame. Rather than tacking on the -10HP system, I had a thought for something I found slightly more interesting.
Alternate Mortality Table
When a character reaches 0HP, they have been dealt a potentially mortal blow and are taken out of action. The DM will consult the appropriate table below, but keep the actual result to himself while narrating the blow appropriately. If the PCs can get the character to relative safety and bandage the character up, then the results are revealed, which may even include that there was simply nothing they could do. If the fallen character is left behind, or the entire party is taken out of action, all characters are assumed dead.
D20 | EffectObviously, this table is mostly particular to melee mechanics, but it's served me very well so far. There's just enough nastiness on the table that being taken out of action is serious, but not enough that it guarantees that it will force you to retire a character.
1-3 | Instant death from obvious source - decapitated, crushed, disemboweled
2-5 | He's dead Jim - Despite the efforts of their companions, they go into shock and 10-20 minutes after the wound is dealt.
5-8 | It's in God's Hands Now - Could go either way as the character bleeds out from their wounds. After d3 hours, they may make a CON roll. If they pass, they survive.
9 | Severe Head Trauma - roll a d4. They lose 2 points of either (1)Dex, (2)Intelligence, (3)Wisdom, (4) Charisma.
10-11 | Sucking Chest Wound - the character is in for a rough night, but will ultimately recover, albeit at the permanent loss of 1 point of Con.
12 | Arm Destroyed - part or whole arm was either sheered from the body, or so badly destroyed as to require amputation. Roll a d6 (1-2) the hand (3-4) at the elbow (5-6) at the shoulder. Odds is left, evens is right.
13 | Leg Destroyed - part or whole leg was either sheered from the body, or so badly destroyed as to require amputation. Roll a d6 (1-2) the foot (3-4) at the knee (5-6) at the hip. Odds is left, evens is right.
14-15 | Limb Fractured/Broken - Roll d4 to determine which, limb is badly injured or broken providing an inconvenience and -1 to any rolls using it until healed.
16 | All Fun and Games - The character loses an eye, reducing their ranged attack by 1.
17 | Ugly Stick - Damage to the face leaving them monstrously scarred. -2 Charisma for reaction checks and hiring NPCs.
18-19 | Gnarled and Scarred - Character earns a few new scars, and will take -1 to everything for the next 24 hours, but comes out no worse for the wear.
20 | Sucker Punched - Character took a blow to the head that knocked them out, but they otherwise have no lasting effects.
My two cents.