All that isn't to diminish the substantial achievements of Diablo, which was quite innovative. The fast, smooth gameplay was novel, as was the setting. It was compulsively playable and accessible while a lot of earlier action RPGs were kind of clunky. The art design was superb -- everything from the dreary, gothic environs to the satisfying animation and sound of a pile of gold bursting forth from a slain enemy. It's fair to say that Diablo was a milestone in ARPG history and highly influential.
Diablo changed things in that it focused on action first, whereas the RPG element is just an add-on. Its gameplay cycles between essentially going deeper in a dungeon that's becoming more difficult, obtaining/selling etc. of items/potions in town so that you can descend further. Its story isn't really of (heavy) importance.