Well I'm pretty proud of myself, I got 9 pages into the thread before I felt I needed to speak up.
I finished ME3, and was very depressed/upset/angry about it, and after reading some people's (SajuukKhar and Synobal's in particular) interpretations/explanations of why it was done the way it was, I can see the logic and sense behind it. I am, however, still depressed/upset and angry about it.
I entirely get that destroying the Mass Relays was the best thing that could happen, and that this opens the door for entire new swathes of progress and self-determinism for every race in the galaxy, and I don't subscribe to the belief that because vast numbers of Turians and Quarians were at Earth at the time of the shitstorm, their races are doomed. It will be interesting, if they make a Mass Effect 4, to see a brand new universe sprung up from the ashes of the previous one, and the ending truly was bittersweet in that sense.
However, what does make me, and I think most people who are upset with the ending, angry is that you have no other choice. Mass Effect, as a game series (not going into the themes and so on here), has always been about (for me) making decisions, which could be right, could be wrong, and in the end you just have to wear it either way. My main gripe with the ending is that it robs me of the choice to make the "bad" decision (i.e. kill the Reapers, the Mass Relays remain, galactic civilisation continues to be bound by the Reaper's technology), and forces my hand, something that a game series based around determining the fate of the universe via your own choices should not have done.
I wanted a cliché happy ending where I see Shepard and Tali living in their home on the Quarian homeworld, and they have Garrus around for dinner some nights. The fact that this ending isn't even a possibility, even if it's infeasible and, in most respects, a bad thing for the universe as a whole, sort of ruins the entire concept of choice that the games were built around.
I wouldn't care if Mass Effect 4 treats the "relays destroyed/everything goes back a few centuries and all the races have to re-discover space travel for themselves" as canon, but for fucks sake, let me have my happy ending. This isn't a novel where I have to deal with the one ending I get given, this is a game, and on top of that, a game that touts choice and decision as defining factors of itself. The fact that the ending flew in the face of all of that is what upset me, anyway.
Just my two cents on why the ending has polarised the fans so much, let me know if it struck a chord or not.