Eh, I found the ending of ME 3 to be rather disappointing. Not just because you couldn't earn a happy ending, but that was a large part of it. The whole game has been, thematically, a denial of inevitability and certain death, and then star child basically railroads you into dying one way or another. Yay. I understood it in Dragon Age - that sort of "someone has to die" was very appropriate there. In ME 3, it was a giant dramatic disconnect. It's a bit like if Luke died in Star Wars:RotJ...all that build up, all the heroism and struggle, and then "oops, guess Vader couldn't change". It would still be a story where the good guys win, but it would suck, comparatively.
Anyway, the thing that really irritated me was the gates going boom and the lack of any description regarding the future. I mean, I've watched the extended cut, and it still sucks. What happens to your squad mates? Does the fleet survive? How bad off is Earth, and the human race? How about the Asari, Krogan, Geth, and all the others we tried to save? Huh? Instead, we get a plaque on a wall, and a couple stills of happy aliens. Oh, goody.
As far as end-game goes, Fallout 2 had a much, much more satisfying end sequence, and it did it without any flashy cut-scenes or action sequences, or even choices. Because they gave you a real conclusion that tied up the threads - not always happily, but reasonably well. ME 3 just left you going "uh, okay, so I'm dead, my friends are sad, and the galaxy is saved. But what happened?"