If you're going to do an X-Men movie, it has to feel like the X-Men. Or why bother buying the film rights in the first place?
These properties established in other media (comics, novels, video games) have a fanbase. And that fanbase is increasingly vocal because of online presence - forums, blogs, Facebook groups, Twitter accounts, etc. Think of Harry Potter and Twilight as well as comics.
There's quite a strong correlation between bad adaptations and bad movies. Marvel Studios has won over the fans by getting the costumes, characters and general 'spirit' of the comics right there on screen. Even though I'm not a fan of all the Marvel Studios output, there's no denying they've found the winning formula. Continuity is good too, another sign they care. VFX are good too.
Fox gets it wrong so many times. Eragon, Dragonball, Fantastic Four, The League of Extraordinary Gentlemen (as well as some elements of the X-Men films). Bad adaptations and also bad movies... because one is related to the other, it shows how much the studio/director/writer really cares.