The D2 fans are softcore, the D1 fans are a little bit more legit, I personally am a Rogue fan.
In Rogue, every game is hardcore. You get one life and when you die, you die. No Tears. No calling Blizzard for account rollbacks.
Diablo 3 has arguably better graphics, but levels feel much more random in Rogue. Each dungeon is a different shape and the caverns connecting the dungeons twist and turn in unique configurations. I never saw the same dungeon twice in Rogue. In contrast, I keep seeing the same backdrops in D3.
Rogue's monsters were made up of letters and the player use his imagination which makes the game timeless, where as at some point Diablo 3 will look dated.
D2 fans love to complain about the AH, the loot, etc. And they're right, but the solution is not to go back to D2 or D1, it's go back to Rogue.