I think the thing that struck me as the worst part of the "ending" was the contrived nature of it. The biggest issue I had was:
The Reaper-controlling child (the ****?) announced that "synthetics will always try to destroy organics." That would have been fine if it had been proposed in the first game, but having it come out of nowhere in the third, especially as I'd spent a good deal of the game helping the Geth and EDI achieve individual sentience and an understanding of the value of organic life, just struck me as idiotic.
Sorry if that hidden tag contains my entire argument, but there's no way to separate it from the spoilers. Not to mention the fact that we're expected to believe that the best outcome for 2 of the 3 choices is that Shepard dies. If the galactic readiness thing had been about whether Shepard has enough time to activate the damned thing and get out alive it would have been far more satisfying. Do all the side quests and get the readiness high enough, and Shepard gets pulled out of the rubble. Skip them to get on with the story and Shepard dies stopping the Reapers. Far more satisfying.
And that bit about it "needing to be a bittersweet ending"; what the hell was most of the game for if not to make your eventual victory bittersweet. How many billions of lives are lost before Shepard can stop the Reapers, or do they mean nothing if the player doesn't personally lose something (I already felt bad when Thessia fell, I didn't need some arbitrary **** shoved down my throat at the end).
Horrible ending to a very good game. Make no mistake, right up until the last 5 minutes it was excellent, full of atmosphere, rich characters, and (relatively) enjoyable gameplay. But the ending has left a very sour taste in my mouth, so much so that it will be some time before I'm willing to play it again, and when I do I'll probably stop before it goes off the rails.