We have Hit Points to give a granularity to character survival, but in a lot of war-games a figure is simply alive or dead – in some, it can be wounded too.
In some RPGs you roll to survive, or against damage. For example, in Shadowrun 2 you get hit and take damage, but roll Body to reduce it. Everyone has 10 HP physical and 10 HP mental, but tougher people are less likely to take damage.
I heard  that early on in D&D development (or maybe it was Chainmail) a figure would die only if it took its HD in damage in a single round. So if your Hero (4 HD) took 3 hits, he would be fine and next round you would still need to get in 4 hits to kill him. That rule didn’t last long, in favor of HP degraded by damage and recovered by healing.
On one hand the “Death Threshhold” mechanic is neat because it makes your character more of a hero, so a bunch of lame darts won’t slowly kill him. But it also makes it so combat is more foregone since a 4th level character vs. three 1st levels can’t possibly take enough damage to die, but will generally slay the three over the course of several rounds. Critical hits could make the combat unpredictable.
On the other hand there’s the resource management of HP to consider: this shifts HP from adventure-level strategy to round-level strategy. Recovery of HP after the fight would give you battle-level HP management strategy.
And maybe you want a little more granularity than OK or Dead.
So how about this:
Your character is either Normal, Hurt, KO, or Dead. If you take your HD in wounds in one round you drop one category. If you take double your HD in one round, you drop two. If 3x HD, just die because seriously you’re outmatched dude.
For example, consider a fight between our 4th level Hero and three 1st level Bandits. Anyone who hits will do 1 wound. A hit is a 50% chance, say 3 in 6. If you roll 6 you give two hits. We’re ignoring armor (say they’re fighting naked in a bathhouse).
Hero has a 50% chance of wounding one bandit, or an 18% chance to knock him out. Each bandit has a 50% chance to hit, but only if two hit and one gets a crit will the Hero get hurt (or a miss and two crits). Pretty much, our Hero will win and probably leave unscathed.
Generally this will keep individuals from killing high-HD creatures. You simply need a mob to take down a 10 HD creature! The same goes for high-HD PCs. Magic weapons and large creatures will deal more than 1 hit, but probably a 10 HD PC will be doing something like 2 or 3 hits depending on what he has. This is where spells becomes really important, since a Lightning Bolt would do 1 hit per level of the caster (save for half).
The rule would be suspended for helpless creatures (for example if a headsman swings his big axe at the exposed neck of the 10th level PC in shackles on the block, don’t bother even rolling to hit – the crowd is awash in blood) and ambushes would give big bonuses to hit.
What about healing? A Cleric would heal you upward one place with a Healing spell as long as he was at least your HD or higher. An 8th level Superhero would need eight 1st level Clerics or else four 2nd or one 8th to heal him. Or possibly a Cleric could blow multiple Healing spells, so a 3rd level Cleric could use 3 spells to heal the Superhero. But do we want healing high-level heroes to be harder than healing farmers? Would they take equal amount of time to heal from resting? If so, shouldn’t magic work on them the same way? And if that’s the case, how would healing magic get more powerful from the basic spell?
I don’t know if I like it. It feels like the regenerating-health-FPS trend lately, which I don’t favor. You get a game that’s pretty brutal on damage and they call it realistic, but if you crouch in a puddle behind a crate your wounds go away. I don’t know if I’d rather spend time managing medical items, or run away to rest up in town, or walk on healing pickups as in Doom.
Again, I guess if you want to minimize resource management it might be a way to go.