I believe that the multiplayer is being made by a team completely seperate from the Single player guys. Come on guys, these people have spent 8 years of their life making this series, I know everyone is still glaring at Bioware over Dragon age 2, but those guys were trying to copy these guys.
Also, with regards to co-op: I noticed people are saying how they would like a friend to tag along as a companion. No-one seems to think that it is just as likely that they will be a companion in their friend's story, with no control while their friend is expanding their own story and deciding the fate of the universe, separate to the companion. I can't speak for anyone else, but I detest that idea. Mass Effect is my story, not a friend's, and I wouldn't ask of someone else to lose their story for me either.
Also, with regards to co-op: I noticed people are saying how they would like a friend to tag along as a companion. No-one seems to think that it is just as likely that they will be a companion in their friend's story, with no control while their friend is expanding their own story and deciding the fate of the universe, separate to the companion. I can't speak for anyone else, but I detest that idea. Mass Effect is my story, not a friend's, and I wouldn't ask of someone else to lose their story for me either.