I loved Mass Effect 3, but the ending was disappointing. After reading some other people's opinions and thinking about it for a while, I think I know why and I think that getting it to a satisfactory place is easier than it seems. There is actually just one reason the ending bothers me. I don't mind how dark the ending was or that Shepard apparently had to die in order to save the galaxy/win the war. I don't think they had to make it that dark, I kind of like the idea of the good guys winning and getting that sappy sort of ending. But there is a place for that dark "there is no good ending" sort of story-telling too. The problem is that the ending abandons two key elements of the game that are closely intertwined: player choice (and repercussions) and character driven story.
At the end of Mass Effect 3, I would be okay with having to make one final choice about the Reapers (kill them/control them/become them) if they really took the time to give me a little window in to how it affected all the people I love. I want to see the galaxy react to my passing/sacrifice. I want to know that it MEANT something. I want to see more than just Joker crawling out of the Normandy with a couple crew members. I want to see my lover react to my death (again, assuming I have to die, cuz I kind of like sappy happy endings) and to know that she's alive somewhere. I want to see that my work in recruiting all of those races helped in some way. I felt like I worked SO hard to get the galaxy ready, but I don't know if it did a single thing based on the feedback the game gave me.
The fix, I think, is easier than re-writing the whole ending, I think. Rather, they could just add a few cut-scenes to show the impact of your choices and what happens to each of the characters you care about ideally starting with any that die, the other character's reactions to their deaths and your death, and ending on your lover, if you had one. I just want to know that my Shepard mattered to someone and that his life had an impact beyond deciding if we're going to move to the "trans-human singularity era". Because, frankly, I don't care about that, I care about the people/galaxy I was trying to save and the individual characters that the writers did an amazing job making me care about and love.