What HP represent
Hit points don't represent skill, luck etc so much as scale with these things.
Every hit deals at least some real damage (evidence: if a high-level fighter takes 100 seperate 1 hp bites from poisonous snakes, he has to make 100 seperate Fortitude saves, not some smaller number).
However, at higher levels you get better at avoiding damage; a 10 hp hit represents less damage when it happens to a character with 150 hp than it does to a character with 15 hp.
This doesn't mean the first character is physically tougher. (Actually, he probably is, but not by nearly enough to explain having ten times as many hit points).
It also doesn't mean that there are seperate hit points for luck, skill, divine favor, physical damage, willpower and so on. That model leads to silly results as soon as you look at it for too long, the fact that the first-edition DMG seems to support it notwithstanding. Luck, for example, does not ablate as it is used, on most understandings of what luck is.
It means the 150 hp character, by whatever means, manages to mitigate some of the damage the 15 hp character would have taken. The correct model is not "hit points = physical damage + louck, skill etc", it is more like "hit points = physical damage * luck, skill etc" - combat skill and so on act as a scaling factor.
I believe that this model has two advantages over every alternative model I have seen: it is (at least tied for) the most consistent with the rules as written, and it is the least weird in terms of what hit points correspond to from an in-character standpoint. The only thing in the game this fails to model is healing spells, which need serious work to make game-world sense no matter what model of hit points you use.
Regardless, I am of the opinion that as long as you have one or more hit points remaining, you aren't too seriously injured; it's only once you hit zero or negatives that you can be considered badly hurt.
(edit - minor corrections only)