In Diablo III, a high-level item is giving you 200+ vitality. I don't know about other characters, but I believe my Monk is getting 2 vitality per level (and there are 60 levels). A _single_ item can give me more of a stat than I gain from leveling all the way from 1 to 60. As such, leveling up doesn't do much other than allow equip items with better stats.
Because items in Diablo II gave, comparatively, closer benefits to gaining another level than those in Diablo III, I felt like I was making progress with either a level up or a nice item drop. If my character was getting decimated in Act IV of Nightmare, I could go back and gain a few levels and maybe get some new items in the process. In Diablo III, not only do your levels not increase your survivability or killing power in any way (besides new skill runes), by the time you feel like you aren't strong enough to defeat an encounter you'll most likely have few or even no more levels left to gain. The only tangible measure of progress is "phatter lewt" at this point.