I have become rather enamored of Swords and Wizardry Continual Light. I've always had a soft spot for S&W, it being my first retro clone. Anyway, I totted up what I think (hope) is a suitable system for critical hits for SWCL, or any other suitably old school iteration of our favorite game.
If damage roll is a natural “6”, roll on critical hit table. If you have no damage bonus, roll d6. If damage bonus (from any/all sources) is +1, roll d8. If +2, roll d10, and if +3 or greater roll d12. If damage bonus is -1, roll d4. If -2, roll d3, and if -3, roll d2.
1 Gain initiative on THIS opponent next round.
2 This opponent is -1 “to-hit” vs you next round.
3 +1 damage from this attack.
4 +1 “to-hit” on this opponent next round.
5 Immediate free attack on this opponent.
6 Starting next round, opponent suffers 1 point of damage at beginning of round.
7 Opponent off balance, loses ability to act next round.
8 Add “to-hit” bonus total to damage from this attack.
9 Opponent prone. -2 AC and must spend next round getting up. Or crawling away.
10 Starting next round, opponent suffers 2 points of damage at beginning of round.
11 Roll additional damage die. If this roll is natural “6”, roll on Critical Table again. Damage modifiers do not apply to the additional damage die, nor to an additional roll on this table.
12 Brutal injury. Opponent suffers -1 to any stat of attacker's choice. This damage requires 6 months to heal naturally. This can be reduced by 2 weeks per casting of Cure Wounds I of per use of Healing Potion. It may be reduced by one month per casting of Cure Wounds II.
Additional Combat Notes
Magic weapons no longer add their bonus “to-hit”. Instead they add their bonus to the wielder's level. Depending on class and level, this may mean that the character is receiving no bonus “to-hit”. Damage bonus remains unaffected.
Potion of Heroism: +2 bonus to Armor Class and damage rolls for one hour. For purposes of “to-hit” bonuses, the character is considered two levels higher for the duration of the potion's effects.
By the way, I know that one of the knocks against critical hits is the ratio of monster attack rolls vs player attack rolls. I suggest not allowing any monster/opponent with 3 or fewer HD to roll on this table. Effectively that means that no low down goblin is going to kill your 7th level dwarf with one lucky roll. I've seen it happen and it was ugly.