I guess one difference that sticks out is that all those X-characters were largely on the same page. In the first movie for example, you had two mostly united groups -- the X-Men and Magneto's band of merry mutants, each with its own overall goal.
Whereas in the movies that have fallen flat, you get a smorgasbord of free agents with competing or divergent agendas. The movie ends up spending more time trying to divvy up screen time to explain the whole fiasco rather than telling a compelling story.
But then there's also Guardians, where they just launched right into it and hashed out all the back story as they went along...
Meh, we'll see. In conclusion, Robert Pattinson sucks.