I've run three sessions of ACKs and so far my favorite sub-system in the game are the rules for dying. The "Mortal Wounds" table would work well in any old school game. I've seen several versions of a "death and dying" table in the various OSR blogs but I find this one to be my favorite.
The basic mechanic is this; when a character's hit points go to zero the character is then incapacitated. When another PC checks that character the judge checks the chart for modifiers that take into account how far below zero hit points the character has gone, the amount of time after taking the wound has elapsed, the character's constitution modifier and the type of healing received. The judge then rolls a d20 and a d6. The judge applies the various modifiers to the d20 roll and looks on the chart to determine of the character is dead or wounded. The chart also tells the judge just how wounded the character is and the d6 roll determines the exact result of that wound. In some cases the wound is fatal and there is little or nothing left of the corpse to be restored. In other cases, the character has only taken a minor wound and will be back in action after a period of rest.
Most of the results require several days to several weeks of bed rest and will leave the character with permanent scars, lost limbs and negative dice modifiers. This result accomplishes several things. It allows combat to have a broader set of consequences. A player knows that if his character goes to zero it means something bad is likely to happen. In AD&D, hitting zero doesn't mean a lot if you have a cleric or a healing potion on hand. -10 means death but there are few consequences for going below zero and surviving. In ACKs, the consequences can be quite harsh. This encourages players to think and practice caution before going into a potentially dangerous situation. At the same time, zero hit points doesn't necessarily mean the character is dead. In several old school games, zero is dead, end of story. This system allows characters to survive but leaves the player with a reminder.
These reminders allow for interesting role playing opportunities and thus interesting characters. A character that has one eye or an arm missing is going to draw attention. Now perhaps the character is known as "lefty" or "One Eyed Jack" in the ale houses of the city. The character may need to get a prosthetic (Hand of Vecna anyone?) or may go looking for some sort of magical restoration to make themselves whole again. Each session so far, has resulted in a henchman or PC being knocked below 0 and the results have been gnarly in some cases. One character was scalped, another lamed and a pet belonging to a shaman lost an eye. The player reactions are a mixture of dismay that their character now has some disability and excited that combat now has a consequence that is more than death or life.