I got an Xbox 360 for Christmas along with Mass Effect, so I haven't played MMOs very much. I played EverQuest II a little, but gameplay there is going to be sporadic for a while.
Mass Effect is a truly incredible game. Its games like this that remind me what is most missing from MMORPGs: story, and the ability to interact with the world DIFFERENTLY from character to character.
As far as story is concerned, some MMOs dish out little tidbits here and there in the form of story arcs. Some games, such as Final Fantasy XI, have ongoing mission-based story arcs with cut scenes that I enjoyed. Unfortunately, in most MMORPGs, the story arcs tend to play second fiddle to levels and loot as the principle means of advancement. And the missions typically have only one way to complete them (in repetitive steps that only work in one proscribed order).
While the world can't truly change in an MMORPG, your character and the way the world reacts to you certainly can. Some of the really great aspects of Mass Effect include the ability to make decisions regarding your character's background. Ramifications of those decisions manifest themselves later. For example, my character was Earth-born and apparently ran with a gang before joining the military. Later, members of that gang decide to try to cajole him into doing them a favor. Decisions I make regarding my characters alignment (paragon vs. renegade) affect the ways in which I can complete a mission. Some fights can be even be bypassed entirely by charming or intimidating the adversary. This adds to replayability because, even though I already know the story, its still enjoyable playing through the game and seeing how things change when I do things differently.
Future MMORPGs need to incorporate more of that kind of evolution into their story arcs. We need missions that alter how subsequent missions will play depending on their outcome and the decisions you made. We need quests that have multiple solutions. Its not wasted content just because one character won't see everything: everyone is going to hit the level cap some day and roll an alt; there should be something other than a different set of skills and items to equip to differentiate our characters. We need something to give our characters more meaning and more uniqueness than an assortment of min-maxed statistics. Mass Effect (and a few other great RPGs in the past, such as Fallout) gives us that in spades.
The Mass Effect universe would make an excellent setting for a massively multiplayer RPG. With Bioware's story writing, the kind of evolving, personalized story arcs they are known for, and this rich backdrop for a setting, a Mass Effect MMO could only spell success. I sincerely hope they use this IP for their upcoming MMORPG instead of the Knights of the Old Republic IP everyone seems to be expecting. Star Wars is so last century. I'm glad to see I'm not the only one who thinks this.
Still, whether its Star Wars (yawn) or Mass Effect, if Bioware can give us the rich story- and character-driven gameplay they are known for from the single player RPGs they've produced in an MMO (a genre that has been notably weak in those areas), that could be truly ground breaking. Anyway, the Normandy awaits...