Video games cannot be compared to a movie or a book. A movie or book is a NON-interactive medium, which the artist/writer/whatever uses to tell a story. A story which you have no part of, where you are just a spectator. If it sucked, its your opinion, others might like it.
Video games however are different. It is an interactive and new medium. Most games follow a straight linear path, where no matter what you did, the ending was the same. Decisions you made doesn't matter at all. This is true for MOST games.
Mass Effect unqiue selling point was all about decisions. Decisions that you make, will affect how the game plays out. This point was further strengthen by the face that decisions you made in the previous game affects the next.
Mass Effect 3 did follow this USP, until the end. The end did not show how decisions I had made over 3 games mattered. It doesn't matter if I saved the council, it doesn't matter if I had my whole crew killed at the collector's base, it doesn't matter if I rally half the number of species in the galaxy. I will still get the same ending. What happened to your USP? Even games like Chrono Trigger had MULTIPLE ENDINGS. Red, green and blue doesn't count as multiple endings.
Bioware had been hyping up about how your decisions matters in this final chapter, how what you did could condemn a race. Their track record was spotless for 2 games and when I started ME3, it was how I felt... until the ending,
When your USP was totally disregarded at the end, people will feel they have been cheated. If I would use an analogy, it would be like, a salesman, selling me a house, telling me I can change everything inside to get the home that I want. After paying for it and customising everything, the government suddenly enforced a law that all my customisation had to go and I had to revert to what they had planned all along.