That's not a Massive Multiplayer Online Role Playing Game, in a way it is and in a way it isn't - we tell the difference between wether the world is always online.
Like Ever Quest or Ultima Online, when you log out - only YOU character is removed from the world, the rest continues.
In diablo 2, you create the world when you create a game, when you end it, the world dies(metaphorically(sp?) speaking) 🙂