I can't stand whining fanboys with an overinflated sense of their own entitlement and I'll always be the first to make fun of them. However, I feels the complaints about the ME3 ending are justified. It's not that it's a bad ending or even a downer ending. It's that it introduces a last-minute plot twist that comes out of left field, is out of tone with the rest of the series, makes no sense, is full of plot holes and offers no closure. There's also the fact that the three endings are 95% similar and you get them no matter what choices you make throughout the game. It's also the kind of ending BioWare promised we wouldn't get. It's never a good idea to promise people one thing and give them something else.
This video from Angry Joe does the best job explaining what's wrong with the ending: