If you want to know why everyone hates the ending, here you go:
<spoiler=This video contains spoilers of the ending, so do not watch if you haven't beaten it already> The problem is that the prevailing theory, the one that the vast majority of the evidence actually supports, is that nothing that occurs after Shepard takes a Reaper laser to the face actually happens. It's all in his mind as he's fighting off the effects of Indoctrination. This mean that the story doesn't really end...since even if you pick the right choice, that just means that Shepard broke free and the Reapers are still on the rampage.
Beyond that, there were WAY too many plotholes and problems in the ending for people to like it.f It seemed like it was just an incoherent stapling together of cutscenes. How did you squad get back on the Normandy? Why can EDI be seen exiting the Normandy if she was in your squad for the final mission and you picked Destroy All Synthetics?
Anyways, watch this video: it does a great job at explaining everything in the endings and pretty much proves that the Indoctrination Theory of the ending is as close as we're going to get for an answer that actually fits.