Diablo is extremely gothic macabre in atmosphere. The music theme of the town of Tristram is remarkably evocative of the dreadful and unwholesome with its acoustic guitar chords and arpeggios accompanied by a wailing wind's whistle. It's a favorite of mine. This sort of grim and spectral ambiance has always fascinated me, from Castlevania to the Book of Revelation, Dante's Inferno and the covers of Opeth albums and the writings of H.P. Lovecraft. Like Anne Rice's Vampire Lestat, I can sink my teeth into this milieu.
It's the dire and woeful without the gore and gratuitousness to completely crush the subtlety that defines the mood. Diablo has plenty of gore, but then, it is an action hack-and-slash full of the rotten, undead, and demonic; in any case, the tone is never compromised by gameplay conventions.
I've fought my Warrior through the cathedral, catacombs, and caves into Hell itself, now at subterranean floor 13. It has ocurred to me that the game is meant to be played through multiple times, new game+ style, retaining character experience and items from one difficulty level through to the next. I've only reached character level 20 or so, and still don't have any gear I'd say even verges on being 'epic.' We'll see what I come across in the final strata of dungeon.
I also haven't decided yet whether I'll continue on after killing Diablo, try another class, or just move on to Diablo II. Hmmm.