Marvel is still pumping out some really good and really fun movies but there's definitely a little fatigue when it comes to juggling all these superheroes to set up all these storylines to connect all these worlds. A movie like Avengers: Age of Ultron was absolutely, totally good! But it tries to do too much without doing enough in some parts.
Plus, as Screen Junkies hilariously points out: all the other Marvel movies that preceded Avengers 2 had bad arse villains that the Avengers could have easily assembled for too! Also, even more bitingly highlighted: making this movie and dealing with its scope probably destroyed Joss Whedon.