In all fairness, the Nephalem thing goes back at least as far as Diablo 2. I think it does do a decent job of explaining why some people are able to stand up to demons (if all it took was some muscle and a sharp blade to take down a Prime Evil, the demons wouldn't be nearly as intimidating).
Diablo 3's problem was making the Nephalem so much stronger than everyone else that the story didn't even pretend like the demons posed a threat. But I do agree that, Nephalem or not, Diablo 2 level heroes work best. Clearly puts the heroes above normal people, but not to the extent that all tension is removed from the story.