Most of the negative reviews I've been seeing have been along the lines of "If you're not into comic book movies, there's a lot to complain about here" and most of the positive reviews have been along the lines of "If you're a comic book reader, then this is for you."
Isn't that what Joss Whedon has aimed to do in the first place? To make COMIC BOOK movies that actually feel like comic books?
A lot of the reviews (positive and negative alike) remind me of Iron Man 2 as well: That the movie serves as a big bridge to the finale that will be the Infinity War. Lots of people feel that Iron Man 2 was just a setup for the other movies rather than being a standalone film. That may very well be the case with Age of Ultron. I don't consider that to be a bad thing though. I actually liked Iron Man 2, and I feel that Iron Man 2 has improved over the years when you view it as a part of a whole cinematic universe rather than a standalone film.