I'm going to go out on a bit of a limb here and hypothesize that Marvel/Disney's success isn't about creativity (though that certainly plays a big part) or faithfulness to the source material (a premise I've always found more than a little dubious, given the cross-cultural pull these movies clearly have) as something a little more uncomfortable- quality control.
Most of the mainstream-oriented superhero films not helmed by Marvel in recent years (setting aside for the moment less-mainstream quasi-superhero offerings like Watchmen, V for Vendetta, Lucy, etc.) have had significant problems. Nolan's Batman was the best of them, and even it had its share of issues, especially in the final offering. When one gets to recent Fantastic Fours and Spider-Mans and Man of Steels, the issues get more noticiable. Villain isn't threatening. Hero isn't likable or identifiable. Plot is cluttered and fragmented. Humor falls flat. Tone is inconsistent in a way that seems unintentional.
The thing is, in many cases I suspect it isn't that the people immediately responsible for acting/directing/writing these films are inherently of an inferior quality to those making Marvel ones. It's that somewhere in Marvel there's a firewall that's holding a line and saying "this isn't good enough, send it back and try again." I suspect that's mostly- but not entirely- at a script level. The quality of seemingly incidental but important things like banter among team-mates and beats between dialogue and action are so consistent among Marvel movies that I have to believe there's some sort of refining process going on behind the scenes, something that gives Favreau's Iron Man and Whedon's Avengers and Gunn's Guardians an appealing kind of continuity that goes well beyond a shared universe.
It's all but impossible to imagine Marvel mis-stepping as badly as the needless, witless fights in X-Men Origins: Wolverine or the stapled-on bad guys of later Spider-Man movies (both Raimi and Webb). They just wouldn't have gotten that far. (Anyone else notice that scene of a topless Gamora in the Guardians trailer got nixed in the final movie?)
My real fear as far as Marvel goes is that this polish will ebb away, especially in the wake of such a full docket of movies to oversee. Other studios have a certain amount of excuse in playing catch-up to Marvel. Marvel is, in a real way, competing against itself at this point. And if whatever forces keep quality in check behind the scenes get rushed or simply decide that product is product and we'll suck up whatever they produce so long as it has the "Marvel" brand stamped on it, it's their race to lose.