It is ME3's decided ambiguity, the deliberate obfuscation, and the fact that it, while telling you it is over, it is also telling you that you can never know how it actually ended. The end is or is not a dream, is or is not part of the game's reality. It is not that it ended poorly, it is that it absolutely decided that you can not know how it ended. Ambiguity that is part of the narrative, is fine and understandable, but the ambiguity in ME3, only occurs in the last ten minutes. It isn't that it is bad, it is that it is decidedly manipulative.
I am also fairly certain that ME3 will have DLC. So the product is absolutely going to be expanded. I am not saying the ending should be part of that, but I am saying that it is not the same as a static product like a movie or a book.