If BioWare drops the ball on ME3, I absolutely will not forgive them. Mass Effect has become their biggest franchise, and they're obviously putting a lot of resources into it, even at the expense of other franchises... *eyes Dragon Age II with utter hatred*
At this point, ME3 has been set up to be possibly the most epic video game of all time, story-wise. Heck, the story that will unfold seems set to rival even some of the most epic films of all time. Unfortunately, this means that BioWare has written a pretty big check that they might have trouble cashing.
My biggest worry is the implementation of past decisions. I liked the idea in ME2, but felt that it was largely superficial and didn't really do much to change my experience on a deep level. Unfortunately, given the gravity of some of the choices made in ME1 and 2 (the rachni, the reaper ship, the geth, etc.), BioWare is going to have to craft one hell of a lot of major branches into the story to accomodate every possible combination. It's an unprecedented undertaking in interactive storytelling, and as much confidence as I have in BioWare, I'm not sure if they can pull it off and still make Mass Effect 3 the epic conclusion that it needs to be.