Diablo 1 was a town on top of a large dungeon. Diablo 2 had multiple towns (one for each of the 4 acts), with stuff to kill above ground while you were making your way to the next dungeon (and more classes, henchmen you could hire, etc).

While it has RPG components, Dragon Commander is an eclectic mix of genres and ideas that makes it difficult to categorize. It's utterly unlike anything else in the series or any of the other major game types (Diablo, BG, etc) though.