I went to see Avengers: Age of Ultron. A few scattered thoughts:
* The primary weakness is that it is overloaded; it is trying to do far more things than it can entirely do successfully. The first Avengers movie managed to work despite a weak story premise by keeping things relatively simple and streamlined -- there was a lot going on, spectacle-wise, but the actual plot and themes were straightforward. This movie does not stay streamlined. This is practically symbolized by the movie itself, which has a plethora of cameos.
* I don't know if it's just me, but the cinematography also seemed sloppier in this one -- there are big flashy spectacle scenes in which it is difficult to figure out exactly what's going on.
* The reason I think it doesn't fail, and manages to make a sequel almost as good as the first Avengers movie, is that almost all of its story premises are better than the story premise of the first one. There's more than one, and one is occasionally left feeling that a good thread in the movie would have been much better if it had been its own movie. But the way in which they are integrated is at least watchable.
* The scene with all of them just hanging out after the party (which comes somewhere in the middle of the movie) is even better than the trailer suggests. Of course, part of the joke (which they put in the trailer) was that Captain America is one of the very small handful of people in the comics who actually is 'worthy to wield the power of Thor'; he has wielded Mjolnir more than once. But it works even better with more context.