The common theme amongst those critics who are underwhelmed is that the climatic, city wide destruction fights are all looking very same-y these days. How true. (Well, I think, since I only get to view them in bits in pieces when they show up on free to air TV a few years later.)
Anthony Lane writes one of the wittiest reviews of the movie, with sections like this:
The story begins with a fight in a forest and ends with a fight in a city that floats in midair. In between, there is a fight in a castle, a fight on a freeway, and a fight in the wake of a cocktail party. The loudest fight is a tussle between Iron Man and the Hulk, which is part of a cunning scheme to rip the Avengers apart. Bring it on, I say. It has something to do with dreams, which are triggered by a blast of hypno-magic from the Scarlet Witch (Elizabeth Olsen), a new player in the game; each hero is disabled by harsh visions, tailored to touch upon his or her worst fears. I vaguely hoped that Thor would find himself holding hands with Hello Kitty, but no joy.
The experience of watching “Avengers: Age of Ultron”—which is not just long but, in Iron Man’s words, “Eugene O’Neill long”—runs as follows. First, you try to understand what the hell is going on. Then you slowly realize that you will never understand what is going on. And, last, you wind up with the distinct impression that, if there was anything to understand, it wasn’t worth the sweat.By the way, showing that he's not just a humourist, Anthony Lane writes well about Gallipoli in another piece inspired by ANZAC Day (and the movie "The Water Diviner".)