ME 3 is a good game, it is not a perfect game and certainly not deserving of the scores some reviewers are giving it. It has quite a few flaws, some of them could have been easily corrected. The cynic in me is saying they focused too much on the multiplayer and not enough on the singleplayer.
For most people its that they spent 100s of hours, trying out different playthroughs and this is how it ends. And that is whats upsetting them