Diablo is about an European tradition, or at least a vision we have of that faded by the time, that's why it's unique
But there are some things I didn't like, for instance the easy way we can gimp our character by choosing the wrong skills. While in the first Diablo it's almost impossible, the same thing that makes characters unique in Diablo II makes we do mistakes over and over. But it's part of the game difficulty, I guess, and a learning curve.
What really made me not give it a prime grade was that this game didn't follow the gothic line the first Diablo went. While in the first we felt alone in that dark European scenario, the second one was too much of a multiculturalist theme. The dungeons to hell were darker, bloodier and lonelier in the first Diablo, when we get to an Egyptian desert it kind of breaks the immersion. Not to mention the third scenario, that rain forest with those comic little men.
Despite that Diablo II is a superb game, it's incredible addictive, played for years... Various other games tried the same formula, but they failed because Diablo II is more than a system, it's an environment, it speaks about our long tradition, and that's why Diablos that are not Diablo don't make the same success. When we say "this game is not dark enough" we are saying "this game don't speak for our occidental European tradition, it's some modern boring thing".