DND_Reborn
The High Aldwin
Well, every game has some way of monitoring damage. You mention HP isn't used for anything else, but condition monitors/ wound levels/ and similar mechanics are really just their version of levels of exhaustion-type penalties or static penalties for each level of sustained injury.What would happen if we took Hit Points out of D&D?
I've had homebrew systems using the Damage Threshold concept where when you took damage exceeding it, you gained a level of exhaustion, similar to something like you suggested.
Personally, I've always liked games like Shadowrun and Vampire. Shadowrun because it had a Stun and Physical condition track and Vampire differentiated between stun (IIRC correctly)/ lethal/ and aggravated damage.
One of the failings of HP in D&D is it is used for all types of damage. Even in AD&D, non-lethal damage only did 25% "real" damage and 75% temporary (faster healed) damage IIRC.
Personally, I like the Vitality / Wounds system of d20 SW, but Vitality is really just hp. However, Jedi spend it for force powers and we toyed with having casters spend hp for spells in a similar manner.
I agree with @Tonguez using attribute damage would be interesting, or any system which handles different forms of damage differently.