What's your opinion on AoU?
Long story short I think there are areas where it genuinely improved upon the first, but it also felt like a retread and didn't quite capture lightning in a bottle like it's predecessor so I understand mild disappointment even though it's still a really good movie in it's own right. I just think it's an easier movie to pick apart given that it's flaws are more readily apparent.
The only problem I had with Ultron is that he comes out of nowhere. There's no backstory, no exposition to set up what exactly Ultron is so his turn towards evil feels rushed and unearned. Tony just says, "this might be the first step in creating Ultron" and we're supposed to just go with it. I liked what they did with him the rest of the movie, and I didn't mind him being quippy either, but a little bit more time spent setting him up would've gone a long way.
The Thor hot tub scene was also problematic and rushed. Within the context of the movie it didn't really make a whole lot of sense.
However, I must stress again that I still think it's a really good movie that's smart, sincere, and self-aware. People knock Quicksilver saying that he was uninteresting and almost a non-entity, but I'd argue that his arc with Hawkeye was extremely satisfying. "Bet you didn't see that coming" is a great line, a great resolution for that character, and a nice wink to the audience.
The Vision is the money-shot. Not only is he one of my favorite characters from the comics, but I love the journey that "Jarvis" has taken in the MCU. This might have nothing to do with AoU, but it's an extremely satisfying through-line that hits all my fanboy sweet spots. Edwin Jarvis was Howard Stark's butler and is brilliantly cast in Agent Carter, who served as the inspiration for Tony's AI system. Now "Jarvis" is a superhero in his own right, and the Vision more than lived up to this legacy in what little we saw of him.
I also did like the often-maligned Black Widow/Hulk romance. It was out of left field but the movie presented it in a way that made sense and they ended it as they should. Overall I'd say that I think it isn't one of Marvel's finest (Avengers, Iron Man, Guardians, TWS, and I'd even argue Ant-man) but at some point "good" is an inevitably with a franchise as prolific and expansive as the MCU, so the disappointment shouldn't be overstated.