RAGE is an FPS shooter and it was advertised as such.
Critical graphical issues on launch pissed a lot of people off who have since not even bothered to go back to the game. Some so pathetic as the whinge about the file size (take a look around this is actually a big complaint - wtf? games aren't getting smaller).
Others, didn't like the story. Were confused that it wasn't Borderlands (or another RPG). Or simply didn't like the art style.
I'm not sure anyone was happy with the ending. We ALL wanted a boss fight I am sure.