On a more abstract level...
It is obvious that the health system can not be too realistic. A sword slashed across one's unarmoured chest isn't fatal per se, but in most cases it would be at least 100% debilitating. You can't do much other than bend over and cringe in pain, with an open wound across your chest. An arrow to the knee and it should be end-of-the-encounter, really. A fireball to the face and it's reload time.
That obviously would mean gameover/scumreload at every other encounter (mudcrabs!). Thus the health system needs to mitigate harsh, pure realism, and to make it so that even at 10% health, a player has a chance (however slim) to win a fight. Because, ultimately, the player wants, and needs to,
win the fight. I personally don't enjoy games where I can roam around wrecking havoc and critting everything to a pulp instantly, and I similarly don't like being unrealistically challenged by a silly critter that I have beaten 200 times already, just because of a "lucky" hit/roll.
A simple HP system can be really detrimental to realism, to many: some players do have a grudge against having 1hp left and still being able to operate at full death-dealing capacity. It's immershun-breaking and stuff. On the other hand, as mentioned, any fight would become nigh-impossible once the player is down to 50% health.
These factors should be taken into account. As I mentioned, a double endurance/hp system (a la PoE) works ok-ish, as does Drakensang's "wounds" system, or Fallout's limbs management. The "bruise" system" mentioned by
thesheeep seems to be partially following the same reasoning too. An inspired combination of these could result in a very good, both realistic and "enjoyable" (eg not overly punishing) health system. I for one would like to see HP actually reflect the character's condition, so 50% health should mean a somewhat reduced efficiency (less damage, less speed, etc): and yet I realize this could greatly conflict with an "accessible" gaming experience.
Of course, different layers of complexity and/or realism can be added to the same health system, via options or difficulty levels.
I think it's up to you (
hiver ), to make your mind up about what kind of game/experience you want to deliver. Keep in mind that you can't just please everyone, make your mind up and stick with the plan.