Gauntlet is an action game, it require mastery of a character's moveset (proper positioning, timing, crowd-control, coordination with teammates, etc.), understanding enemy behaviors and encounter design, constant on-the-fly situation assessment and being able to adapt your tactics accordingly (which ties the first two points together), and non-stop aggression. I haven't played Diablo III, so I can't comment on it, but Diablo I and II were neither of those things, and not even good games, for that matter. They're just grindy, item-collecting, click-a-thon 'RPGs' -- utterly mindless games.
I can understand liking the latter more, it's easier to play and if you're into things like 'narratives' and 'lore', than the Diablo games definitely offer more on that front, but I'm really not seeing the straight comparison people have made in this thread. Is it because of the top-down camera and fantasy theme? I suppose Chrono Trigger (and other JRPGs), the fantasy-themed Harvest Moon games, and 2D Zelda titles are also the same thing. We can probably also throw in DMC, Warcraft, and some fantasty-themed city building game!