To sum up a review of Avengers: Age of Ultron in one sentence: It was great like the first one, just not as great. What was the primary complaint of the first film? For many it was the opening sequence. It felt rushed, choppy, and incohesive with the rest of the film. It was like Joss Whedon hadn't had his coffee yet for the prologue. The film quickly picks up rhythm after that scene and Whedon finds his groove, quickly entrenching you in the film.
The problem with AoU is similar to the opening sequence of the first film, but instead it is sprawled across the entire film. Whedon retains his usual charm and passes it amongst the characters he has built for the big screen, but it is blended throughout with the same jagged edges the first film opened up with. The pace gets choppy, motives are present but not fully developed, and explanations are dropped as quickly as Tony Stark's quips.
You can point to a couple of reasons for this strain on the story. Firstly, the introduction of Vision in a manner that would be conducive for both fan boys and casual movie patrons proved troublesome. What birthed from this attempted union was a convoluted and rushed subplot that felt more like Raimi's forced addition of Venom in Spider-Man 3 than the natural progression of a character. Secondly, the "uneven character development" complaint from the first film was remedied in the sequel (particularly for our two "normies" Black Widow and Hawkeye), but not without sacrifice to pace and structure. It is the price you pay to balance so many personalities at once, but it was a valiant effort that didn't subtract too much from the overall quality of the film.
AoU was not a failure. It was as fun as the first, having moments of levity to toss around the scenes of pensive gloom. Ultron was a blast, and it was nice to see an original villain for the group to battle that wasn't a repeat from a shared franchise (sorry, Loki). Ultron's character was a perfect blend of daddy issues and genocidal narcissism, and Spader pulled it off perfectly. The only complaint I can share with the character (SPOILERS AHEAD) was his briefly-lived boast of strength at the end. Like with Loki from the first film, Ultron's strength was in his numbers, and while he could take a punch much better than Loki, he was far too easily dispatched at the end. Like the meteor they fought on, I expected a more epic conclusion to his short-lived reign (Final Fantasy VII, anyone?).
Obviously, AoU isn't horror. We have bent that rule often here, and perhaps too much, but it is hard to pass up on reviewing a massively successful film that covers the other portion of our persona; comics. Few films will hold par to their epic predecessors. It is not always a good enough reason to scrooge a film franchise we love when it falls short of its previous installment(s), but simply praise the films that somehow pull it off.