X-men FC can take a page from 2 other comic book movie failures:
Watchmen fell short for a couple different reasons (read Anderson's breakdown for more), but one of the biggest ones that no one cared about the characters. I think most "normal" people recognize Cyclops, Jean Grey, Wolverine and Storm from the original movies. Instead of cashing in on that familiarity, you're going to introduce all new characters that NO ONE knows anything about (including some comic book fans, me included). Havok, Banshee, Darwin, Riptide, Azazel....a lot of head scratchers. Unless you make a 5 hour movie you're not going to be able to set up all these characters in time enough that anyone will grow attached.
Superman can kick some ass, but you wouldn't know it from Superman Returns. We spend more time with Supe's emotional battles then we do seeing him putting a whoopin on anybody.
X-men FC is going to show how Xavier and Magneto
I am all for plot/character development and I hate fighting and explosions for the sake of making a trailer look cool. That said, if you're making a comic book movie you have to bring the goods.
I liked the original trilogy of X-men movies and I would love to see more great X-men movies. Hopefully they're building something cool and I'm off the mark. Maybe those old characters need to step aside for a new team. We'll see.