D2:ED was originally going to have a multi-player component, but during development Larian dropped it as it was placing too many restrictions on quest design, etc.
This.
I do not want the single player mode to be reduced to accomodate multiplayer (as I usually play in single player). However, if they have a level / campaign editor then it might be nice to be able to be able play modules made for multiplayer some times (separate from the main game).
