I had managed to avoid all spoilers of the ending, so I managed to get a first hand impression on it, and I'd like to make a few points.
The ending itself, in terms of the plot and the content, wasn't a problem per se. If it was fleshed out a bit more during the game, and during the final cutscenes, it would have been a great ending. The plot itself actually dealt with very interesting ideas and points, like are synthetics inherently rebellious and can they co-exist with organics, or even just the relationship between masters/those higher up the food chain to servants/slaves (Prothean goes into that a bit, if you have him). As it is, it's only a decent ending, and certainly fell short of expectations.
But the real problem isn't that the ending was too sad or made no sense (and honestly, an ending was too sad? Crap's hitting the fan in the galaxy, it's not gonna be pretty and rainbows. This isn't a cause for complaint =P). As mentioned by other posters before, it is the execution of the ending that was the real let down for me. If you've studied Shakespearean and Greek tragedies, the stories always put the hero through trials and sacrifices, and culminates in a final release of emotion for the audience at the conclusion of the play. In plays like Hamlet and Othello, we travel with the tragic hero, understand his motivations, build up a connection with him emotionally, and at the end, this is all released. Explodes, even.
The problem is, this doesn't happen in ME3 (at least, not for me). The audience (in this case, the gamer) doesn't get a cathartic moment. He or she doesn't get that release of emotion which has been building up from somewhere around ME2. You invest in the characters in ME1 and 2, and you watch some die along the way. And in ME3, you watch some make incredible sacrifices for you, the hero, to get to the end. I will never ever forget Thane's final fight and death to my last breath; that scene alone brought home to the gamer what this was all about, and that you better get to the end and get this crap done. Throughout the game, you FEEL it when those around you that you cared about, like Legion, like Mordin, like Thane, fall down in sacrifices for YOUR cause. I teared up every time one of them fell. But all this emotion is built up to the final moments where...... well, what happens, exactly? There's no release of emotion, either sadness or happiness. It just kinda...... ends.
The game itself was utterly fantastic. The fights were insane, the plot moved quite well, and at all times I felt the urgency and the desperate nature of the situation. But I wanted a cathartic moment at the end. I didn't care if it was happy or sad, and quite frankly, having built up my sad emotions at every stage of the game, an extremely bittersweet ending wouldn't have been bad at all. In fact, I'd probably welcome it. I just wanted that release.
Man, that was long. I guess this is my 2 cents worth.
TL;DR --> The problem with the ending wasn't that it was too sad, or too what the hut. It was the fact that at the end, we the player did not receive that cathartic moment we all so craved.