So, somehow I never played the original Diablo or D2.
Is it just me, or is the equation
Diablo = (Nethack - Quirky Item System - Insane Difficulty) + ([Insert Fighting Game] - PVP) + Story + Quests
Approximately correct?
+1 for nethack!
Diablo is light on quests relative to more formal RPGS... although there is a story you are working through.
Diablo is a gothic themed slot machine, where instead of pulling a lever, you kill monsters, and instead of getting money, you get items.
The "game" is basically trying to figure out out the best skills to use to maximize the amount of items you can get for the minimum amount of time.
There is a story - and it looks like it takes between 8 (really rushing) and 20 (really playing slow) hours to complete. Once you finish it, you play it again on a harder difficulty (nightmare), and then again (hell difficulty), and then one more time (inferno difficulty). Blizzard claims that inferno is so hard that none of their internal testers were able to beat it.
Once you're done, then end game is usually about some manner of item hunting runs. (The rewards of which are better items which make future item hunting runs faster!) They've allegedly tweaked things a bit to make it more interesting than D2 end game, but I don't know that we'll know this for sure until people are actually at the end of the game.
I love it, and nethack, but they scratch different itches. You'll never die by falling down the stairs and having your cockatrice corpse weapon (wielded with gloves of course) kill you in Diablo. Angels however, are lovingly rendered in 3d.... vs. ascii character 65.