Call me highly optimistic. If the Multiplayer is something like deathmatch, capture the flag, or anything like that, then I will be somewhat sad (but that's because I'm not really into those type of modes). If its similar to Horde mode, then that's just awesome. Co-op? As long as its a separate mode (as I don't really see it working out too well in Single Player) detailing some events that happen during the story that we might not get to see (maybe even some more appearances of characters that may only be temporary squad mates in the single player portion)then that's just as good. You don't even have to play as your Shepard for any of this. Maybe just random Batarian Soldier, or some Turian biotic, etc. This way they can totally explain the balance issues that will occur with the classes if they decide to implement that type of feature.
Honestly, it all depends on how Bioware handles it. The multiplayer/co-op makes sense given the setting of ME3, but if it ends up actually feeling like I'm playing online as opposed to something that I can see happening in the universe/story, then I'll probably be a bit put off.
Either way, I'm going to get this game. I'll just need to wait a bit more on how these modes work and actually play. Not too worried. At the very least I'll try the multiplayer out.