Not knowing what it is actually called?
Dammit, beat me to it.
Well, it's already been stated and discussed through and through, but I guess until there is something new to discuss about ME:N this what will always be on the table... Ending topics!
Ending could've been better. Given the talent and creativity that I've seen from Bioware's past works (and current!) they could've put together a more solidly, constructed ending. I still believe development woes forced their hand to release what we received.
I still hold the belief that ME2 elements should've been switched with ME3... The crew in ME2 should've been in ME3 because then ALL those new people introduced in media res wouldn't have been an afterthought more or less like they were in ME3. Plus Bioware stretched themselves thin with the import feature with so many characters to account for from ME2. Put them in ME3 for the endgame and then they wouldn't have to worry about the next import (or maybe we will with ME:N ;-D)
ME3 only introduced 2 new characters in Javik (optional) and James. EDI is new too, but obviously would need a rewrite to explain my hypothetical scenario here. So that would've made for an easier transition from ME1 (that familiarity with past SM) as well as making it easier for an import.
Then the Suicide Mission... That was a way to end a game. Hell Bioware didn't want to have a final boss because it was too "gamey"... Cool, I can respect that. Some of the deepest, greatest moments from the ME series did not involved me taking out my rifle once and going pew pew. So make the Catalyst confrontation at the Collector (Reaper) Base.
That really is what felt lacking during ME3's ending to me personally. It didn't have enough build up and reward for all your previous hours of gameplay endeavors. Having a Suicide Mission style ending during Priority: Earth would've been great to see or again, my suggestion of switching ME2 with 3 and having a small group infiltrate the Reaper hive like they do with the CB. That would give me a finality that I can set my watch to.
Aside from that, the Reaper motive, Catalyst, and all that jazz, I'm cool with. I would've liked it presented differently, for example instead of a kid manifestation, do what Levi did and take the form of fallen squad mates, Anderson, TIM, VS, hell anybody. That would've resonated with me more.
So really, I don't have a problem with the story aspect of the ending, save for a few cosmetic changes I'd preferred, it was mostly the final missions that needed an overhaul to send off one of the greatest series in gaming history.