The other answers have all brought up highly relevant points, but I’d like to note a few things, more from a creative perspective.
All superhero films are formulaic, to a degree. Danger emerges, hero rises to meet the challenge, hero fights villain, and, perhaps through some horribly convoluted slight of plot necessity, hero ultimately wins the day.
Of course, Marvel’s better than that. They’ve been making movements to dabble with other, well-established movie genres in interesting ways that also serve to accentuate the merits of the superhero element in film. Examples include:
Captain America franchise
The First Avenger — In the style of a war drama
Winter Soldier — A self-proclaimed “political thriller” reminiscent of the conspiracy movies of the 70′s
Civil War — More of a “psychological thriller,” also as noted by the Russos.
Ant-Man — Very much a heist film, including the traditional three-part plot of plan, execution, escape. But with shrinking, and ants!
Doctor Strange — Essentially the lovechild of Batman Begins, The Matrix, and Inception, from what we’ve seen thus far. A “mind trip” film.
You get the idea. This certainly helps to mix up the feel of each installment a bit, which is no longer as reducible to the classic hero vs. villain plot associated with the classic superhero narrative.
Also of note are tonal shifts in the superhero ‘genre’—ones reflecting satirical attitudes, or even disillusionment, with the moral values and ideals upheld by the standard super story. One might look to films such as:
Guardians of the Galaxy — Which, though preaching superhero morals, took a far more comedic spin on heroism. Its slogan was “all heroes start somewhere,” and played on the idea of a group of unqualified cosmic misfits tackling evil in a far goofier way. Almost anti-heroes, to some degree? Just way less edgy and cool. In many regards, it’s almost a parody of the serious space opera films (pic related):
Civil War — I’m not claiming...