I've mentioned a Spelljammer game a couple of times, and it has already begun. (Watch this space for updates if you're into that sort of thing.) Anyway, since this is first-level characters in AD&D, I decided to make a hack to make combat slightly less deadly, but at great cost.
So You Should Probably Be Dead
As soon as a character hits 0 hit points, that character typically dies. (A variant rule lets characters survive up to -10 hit points, with critically wounded characters worsening by one hit point per round until death.) This rule exists somewhere between those two.
Under this rule, when a character hits 0 or fewer hit points, that character can still be revived as long as a would-be healer gets to the character within one round. (Keep in mind that AD&D rounds are a minute long, so that should be plenty of time; the DM can modify this as necessary.) As long as any healing brings the character above 1 hit point, the character is revived.
Of course, it's not that simple. The character took a shock that most people would not survive, and in fact, that probably required magic to save you. You should be dead. As such, you probably took a serious, permanent, and crippling injury.
As such, any character so revived needs to roll his or her System Shock percentage (DMs who don't use System Shock may take a CON check or a saving throw vs. death or whatever appropriate check you can contemplate instead). Success indicates that hit points are healed, the character is revived, and everything is okay. Failure means that the character rolls 1d20 on the Injury Table, below. Negative hit points are applied as modifiers to the roll below, so a character at 0 hit points takes no penalty, a character at -1 hit points takes a -1 penalty to the d20 roll, and so on. Simple, right?
Injuries on the table below can be healed by way of a regenerate spell or local equivalent, when appropriate. (These injuries probably also disappear on subsequent resurrections, as well.) The DM is the ultimate arbiter for what is required to heal such an injury. (Likewise, the DM can always a declare a character beyond saving through this method. Death from poison, massive damage, decapitation, and suchlike are viable choices. In such case, only raising the character from the dead will work.)
20: Only a Flesh Wound: No further injuries beyond hit point loss.
19: Lingering Scars: No real mechanical injuries, but the character suffers from chronic pain. She can probably predict when it's about to rain with reasonable accuracy.
18: Disfigured: Your scars are significant enough to influence people's opinion of you. Take a -1 reaction adjustment penalty.
17: Broken Limb: Randomly determine a limb. That limb is broken. A broken arm prevents the user from using that arm for anything and temporarily removes 1d4 Strength from the victim. A broken leg reduces the user's movement speed by half and temporarily removes 1d4 Dexterity from the victim. Lost ability points return at a rate of 1 per week assuming the limb is completely immobilized. Failure to properly rest the limb while it heals assesses a permanent -1 penalty to all actions with the limb (and improperly healing a leg means that the victim always treats her encumbrance score as one category worse than it actually is for the purposes of determining speed). Assuming the limb heals properly, it is thereafter treated as a Lingering Scars result.
16: Gelded: The character's reproductive organs are damaged, and the character is now sterile.
15: Injured Larynx: The character's larynx was injured. The character cannot talk above a whisper, and cannot shout. There is a 10% chance that the larynx is injured beyond any use, meaning that the character cannot speak (only making quiet, rasping noises) and cannot cast spells requiring verbal components.
14: Aphasia: The character has difficulty communicating with others. Lose 1d4 Charisma.
13: Delirium: The character suffers from chronic confusion. Lose 1d4 Wisdom.
12: Brain Injury: The character suffers from intelligence loss. Lose 1d4 Intelligence.
11: Injured Spine: The character suffers from balance issues and loss of coordination. Lose 1d4 Dexterity.
10: Gut Wound: The character suffers from a devastating wound to the torso. Lose 1d4 Constitution.
9: Bad Back: The character suffers from a muscular dysfunction. Lose 1d4 Strength.
8: Missing Eye: The character suffers from a -1 penalty whenever depth perception is a useful element (such as melee combat), and a -2 penalty whenever depth perception is vital (such as ranged combat).
7: Ruptured Eardrums: The character is totally deaf.
6: Maimed Limb: As Broken Limb, but the condition is permanent.
5: Blindness: The character loses all eyesight, acting as if in total darkness.
4: Punctured Lung: The character loses 1d4 Constitution, and has difficulties with exertion. Overland movement is half normal, and any period of strenuous physical activity (maybe a minute or so) requires a Constitution check or else the character loses a round catching her breath.
3: Damaged Heart: As Punctured Lung, except a failed Constitution check requires an immediate saving throw vs. death or else the character suffers an immediate and fatal heart attack.
2: Retrograde Amnesia: The character loses a level. Experience points and hit points remain, but the character acts in all other respects as if she were one level lower than she actually is.
1: Coma: Your character revives, but falls unconscious immediately. Assuming proper care and nourishment, the character can make a Constitution check every 1d8 weeks. Failure indicates the coma persists, while success indicates the character awakens. A character loses a number of ability score points equal to the number of weeks spent in the coma. These points should be taken from random ability scores, as determined by the DM.
0 or below: Death: Your character is dead. A generous DM might rule that, since you were revived and died again, you rouse enough in your final moments to give some final words to your would-be savior. A grim confession? An enigmatic statement? A touching memory? The choice is yours.
A Note for 4e Fans: Parties interested in grievous injuries for 4e should consult the Less Death, More Danger! article from Dungeon 204.
Second Note: One of the resources consulted for this post was this Giant in the Playground forum post for 3rd edition house rules regarding maiming. It talks about wounded characters in general and has a much simpler injury table than is recorded above.