If you look at other genres most of them have gone through a lot of changes - FPS is a good example. Back in the day we had Doom, but from then on a lot of things happened, like Half Life, System Shock 2 or whatever. Diablo is a dressed up gauntlet, and gauntlet is an arcade game, basically "Doom" in the RPG world. If you like that that's fine, but for me its lame in the day and age when RPG games offer coherent storylines, involving quests, sometimes diverse gameplay and the like.
It basically takes the worst, most mundane aspect of role playing games, the mechanical grind and progression and makes it the sole virtue of its gameplay.