I predict what I'll find most challenging about MP combat is not being able to pause. I pause a lot, guys 
Yeah, it was a little jarring at first, going from the ME3 campaign to MP, because I also pause a lot. I play an insanity vanguard, so being able to pause-charge-pause-repeat was always so useful, because I could plan my next target or escape route, and also so I could direct everything and everyone else. (I did the same in DA:O as well, although I'm finding that in DA2 my playstyle is closer to that of how I played in the ME3 MP.) So the change was quite jarring, and until I got used to it I died a lot.
But when you have a really good team with you, it gets easier. You don't all have to be really crazy good or anything, but being able to communicate REALLY helps and improves everyone's experience. You can watch each other's backs and catch dangers that you would have caught in the SP during a pause, for instance. "There's a phantom right behind you, look out!" Or I suppose in DA:I's case, it'd be something more like, "Rogue on your back! Don't worry, I've got it!" or, "Can someone please cover me while I try and pick this lock so we can check out that loot?"
So it's good that the people in this thread are excited to play together. I find that multiplayer, no matter what game or what kind, be is PvP or co-op, is always more fun with friends. But that's a given, I suppose. :S