Hit Points.
So, tongue in cheek for a moment. Dimension20 addresses this, in-character. As in, its a high school for adventurers, so the kids 'level up' and get higher hit points, harder to kill, etc. But one of the characters has a father figure that's truly just one of the random NPC's. He's like... a level 3 diplomat. 10 hit points. In season 2 the kids leave their usual 'realm' to go do a quest thing, save the world, etc. They can have three 'hirelings' with them. One of the ones chosen is this father figure. First fight? Dude dies in one hit, first round. Full on dead. He gets revivified. Second fight. Dude dies again, one hit. 10 hit points, y'know? And these kids are like 7th level whatevers. So the stuff they routinely go up against is dealing like 13+ hit points of damage in a single shot. So they revivify him again. Same fight? Dead in one hit. Revivify at end of fight.
Cue confused conversation between daughter and father figure as to his apparent inability to handle a fight. And his freaking the fuck out and yelling at her that he's JUST A GUY. He's a normal, average person. That not everyone is as tough and resilient as she and her friends are. He never went to 'adventuring academy', he went to a SCHOOL.
Anyways, I really liked that idea. That adventurers are these kinda.. freaks of nature that somehow manage to get harder and harder to kill the more you throw at them, so society has just sort of evolved to put them in their own little place to learn their weirdo adventuring stuff, and go 'save the world' while the rest of the society just goes about its business.