Personally I did not like Torchlight when I played it, and I played Diablo II for eons. It was basically one of the only games I had for many years and I really gave all that bottled up sense of exploring to it. I once even got out of bed so fast to play it that I cracked my neck pretty bad! But I digress.
I hold that Diablo 2 has a lot more to it than "addictive gameplay" and "mindless fun". In fact, some nice man on the internet made a video regarding my points (
very slight spoilers within)
I love the cinematics in this game, the story, the atmosphere, as much as the diverse gameplay itself. I do think in terms of the hack and slash / "lootgame" genre it is by far the best ever made. Diablo 1 is fine too, but I find it generally a lot slower and much less atmospheric. Regardless though, I know people have really gotten into Torchlight, but I think you'll be surprised by what Diablo 2 offers. Maybe annoyed at what it
doesn't offer, but I think it's less of a flaw and more of a "slightly inconvenient difference".