I like mmorpg's well I like the basic idea altho none of them have really met my expectations of what a mmorpg should be.
Now I don't wish DD2 to be mmorpg, would be cool tho but to my experience the mmo in mmorpg make it rpg, this works great without decent storyline, repetetive quests, no-one wondering how ridiculous it really is that 234343 poeple before you killed mr badguys leader.
Other than that, mmorpg's require servers, security, gm's etc etc, lots of work Larian can't spend in original content.
Tho, to get to my point a simple co-op mode would be great, accompagnied with a little friends client-server system. A la Diablo II. A more worked out meeting place "say, all inn's act as some cross-client meeting place" would be nice, to have some roleplaying interaction with strangers. But this might not be feaseble in a simple manner.
and ofcourse modability.