The X-men were interesting,but not because they fit the mold of the superhero genre. They never fit the super hero mold. That's the Avenger's niche, which helps in movies but has made their comic awful since issue 1. The X-men were interesting starting with ANAD X-men because they were interesting characters who were further developed in the background of sci-fi/superhero stuff. First class stayed true to what made the X-men interesting: Interesting people in conflict who happen to have super powers.
In other words, the X-men would be interesting even without super powers because of the character dynamics. The Avengers are vacuous even with their shiny super powers.