I was going to post a blog entry on why the Mass Effect 3 ending didn't work, but I came across two thoughtful and well-written editorials on the subject that explain it far better than I could.

The first is this video by Angry Joe from That Guy With The Glasses:

And the other is this article by Becky Chambers at The Mary Sue.

The Angry Joe video has spoilers while The Mary Sue article doesn't.