One topic that seems to come up every so often is that hit points/ health/ morale are kind of absurd. Whether or not you agree with Wilhelm that they are especially absurd when using a blaster, I think we can all agree that the ability to withstand multiple lethal attacks is a fairly baldfaced sacrifice of realism in favor of gameplay. It's one of those things like level based combat and the fact that death doesn't actually kill you that we all just kind of accept as necessary evils.
I've always thought the concept of hit points was a bit silly, even back in my PnP RPG days. In D&D, the maximum damage that a longsword can do, barring strength bonuses and such, is eight points. So if you stab someone in the heart or cut their head off with a longsword (i.e., do as much damage as it is possible to do to someone with it), that equals eight points of damage. So how on earth does anyone get to have more than eight hit points? Gary Gygax explained that all the damage that you take until you get down to your last few HP represents your character getting minor wounds or becoming exhausted. Only once you are worn down do you become vulnerable to a fatal blow. However, that always struck me as a pretty weak-sauce attempt to justify a mechanic that existed purely for (perfectly obvious) gameplay reasons. It also fails to explain how a character can end up with enough hit points to stand in lava for a few rounds.
I used to to mess around with the concept of hit points in my campaigns on occasion. For example, I once ran a Dungeons and Dragons campaign where most normal humans couldn't ever have more than nine hit points (1d6+3 for a high con bonus). In order to have so many hit points that a single sword stroke from a strong guy couldn't kill you, you had to either be a lot bigger than a human, a magical being, or a individual with some kind of divine power. Player characters were not normal humans (or elves, or gnomes or whatever), they were "annointed" under the protection of a deity that had some purpose for them. As you went up in levels you became literally harder to kill. If someone hit you with a sword for eight points of damage, it would go right through your chest and you would laugh it off like a vampire in a horror movie. All your wounds would heal nearly instantly, at least until you had regenerated all the damage you could take (i.e., used up your hit points). I'm not aware of too many MMOs that have used this justification for health bars. Rift, at least, does something similar.
For better or worse, virtually every RPG online or off has something like hit points. Both PCs and mobs can generally endure multiple seemingly fatal blows (weapon hits for max damage, grenades, lightening strikes, and the like). Further, I don't really see any good alternatives. When I envision a RPG where everyone and everything can die in a single hit, it doesn't sound like a lot of fun. It sounds like a game where you spend more time in the graveyard than playing, and one where a level 70 character doesn't feel a lot more powerful than a level one character.