BIG WORDS at the start of the Hit Points section:
Hit points represent your vigor to avoid actual wounds.
When an attack deals hit point damage, if you still have HP afterward that attack only grazed you, or it caused pain that can be overcome. When an attack reduces your HP to 0, it knocks you down and leaves you unable to keep fighting. You're disabled until you either regain hit points or you die. While disabled, you can take no actions but you are aware of your surroundings.
You cannot have negative hit points. When you're out of HP, further damage becomes critical damage. If you have any critical damage, you're wounded. If you have critical damage greater than one-quarter your normal maximum HP, you're severely wounded. Once you take more damage than half your maximum HP, you die.
Getting Back on Your Feet
Various effects can let you can regain HP, up to your original maximum. Most often this represents you getting your second wind and rallying your strength, or an ally inspiring you to keep fighting. Sometimes these are spells that physically heal minor injuries or infuse you with vigor. Once you have any HP you are no longer disabled.
When you take a short rest (5 minutes) you regain all your HP. When you take an extended rest, reduce your critical damage to 0. However, this does not remove the wounded or severely wounded conditions. Those have to heal on their own.
Wounds
When you are wounded, you take a -2 penalty to all d20 rolls and you grant combat advantage. While severely wounded, you take a -5 penalty to all d20 rolls, you grant combat advantage, and you can only take one action per turn.
If left to natural healing, make a DC xx {{Endurance/Constitution/whatever}} check each day to remove the wounded condition, or a DC yy check each week to remove the severely wounded condition.
The Heal skill can let an ally treat your wounds so you heal faster. Some magical effects can remove the wounded condition in just a few moments, or reduce the severely wounded condition to just wounded. The availability of magical healing depends on your setting. Classic D&D makes healing plentiful. Low Fantasy D&D requires long rituals to heal wounds. Grim D&D has no magical healing.
Optional Rule - No Wounds
Some gamers prefer simpler rules. You still die when your critical damage is equal to half your normal maximum HP, but you never become wounded.
Optional Rule - Gruesome Wounds
When a critical hit causes you to become wounded, make a save (DC xx). If you fail, you suffer a gruesome wound appropriate to the attack. These wounds should be something that won't end your adventuring career. You might lose a hand or an eye, but not a whole limb.
If the crit caused you to become severely wounded, the wound should be even more gruesome, of the sort that renders you almost incapable of adventuring unless you can receive magical healing.
My suggestion for the negative HP system is close to this.
Hit point damage represents minor cuts, light bruises, loss of stamina, the drain of combat focus. Temporary hit point represent vigor and confidence that is on top of their normal skill.
A character is
bloodied when they are hit with a critical hit, take massive damage, or drop to 0 HP. Characters can’t be reduced to negative hit points. 0 HP is the minimum. Instead, any character who takes damage that reduces his hit points to 0 must make a Constitution/Endurance save to avoid being
disabled or
dying.
Disabled characters are hurt badly but still able to function somewhat. Dying characters are so hurt badly they are unable to act. Disabled and dying characters are people with concussions, heavily bleeding wounds to the limbs, or minor broken bones to various degrees. A Con/Endurance check is needed to see if they get worse (after every action for the disabled, every round for the dying).
A Wisdom/Heal check can remove the
disabled or
dying states. Another check is used to remove the
bloodied state. Long term rest allows the character a Con/Endurance check to remove the disabled or bloodied state.
Certain spells can remove the
bloodied, disabled, or
dying states.
Then each class can heal differently.
Clerics would have the most powerful hit point damage heals, be able to remove bloodied, disabled, and dying states, and grant temporary HP.
Druids would be the same healing spells as Clerics but use a spell slot one level higher.
Bards can use some spells to heal damage and use songs to grant a lot of temporary HP.
Warlords grant a whole lot of temporary HP but not heal actual HP damage. Some are trained field medics and have a bonus to the heal skill.
Paladins can heal via Lay on wounds but can't grant temporary hp.
Rangers and
Paladins that choose to learn spells can heal HP.
Monks can heal HP damage themselves outside of combat via meditation.
Wizards and
Sorcerers can only grant themselves temporary HP.
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