I suspect that what we're going to see develop is something fairly close to Mass Effect 2's implementation. Except that the loyalty missions from ME2 will actually be helping out your friends with their own class missions.

In short, you'll have your own story missions to work on which can skew your story in one of several directions. In the points between your own story missions you'll be helping out your friends with their own story missions. Those will be able to go in different directions as well. By the time you and your friends make it to the conclusion of the story (or at least, the launch portion of it) you will have had a game experience that is uniquely yours. It will have elements that are similar to everyone else's, but because each of those elements have different possible outcomes you can be sure that your own particular story is quite unique.

This is where it could actually exceed Mass Effect. Sure, you may have three possible endings to the storyline based upon your actions, but you will also have been involved with a few friends who each have their own very different endings of their own.