Jim Raggi over at Lamentations of the Flame Princess has just presented an idea that I rather like for determining how much of your character's hit-points are represent the amount of actual physical damage you can sustain and how much is luck, favour of the gods, plot immunity and so forth.
To summarize, at character creation you'd roll hit-points as if you were a zero-level normal, including any CON bonus/penalty (minimum of 1), and the result is your Actual Physical Damage Hit-Points. Those points take longer to heal than normal hit-points, while normal hit-points (always lost first) recover a lot more quickly (Jim suggests 1d4+CHA or WIS modifier per day). If you go below 0 on your APDHP, you're dead.
If one wanted to make combat more deadly, then natural-20 critical hits could come directly off APDHP, by-passing normal HP entirely. Personally, I've moved away from that sort of thing, but whatever floats your boat.
I'd also like to link that result with the character's physique, so a character with few APDHP would be tiny and frail-looking, while one with many would be huge and hulking. I'd probably want to throw the character's STR into the mix there as well, to avoid those odd tiny-and-skinny-but-infeasibly-strong results.