Ensemble pieces are hard.

Rating: 6/10 (25 November 2019)

You have several main characters revolving around one goal, but all of these characters should be unique and driven by their own wants and needs, but all of those wants and needs need to be connected to the core of the story. When you take half a dozen heroes who are (largely) off on their own adventures most of the time, it can take some doing, and I don't think that doing was done quite well enough here.
The Avengers have been wiping out Hydra one base at a time for some months and have come across the final large stronghold. They quip their way to victory, discovering Loki's spear from the first Avengers as well as two "enhanced" individuals (one who's fast and the other that's weird). With the spear, Tony Stark and Bruce Banner begin to analyze it in order to figure out how to take advantage of it. What they end up doing is creating Ultron, an advanced AI that takes a cryptic mission from Tony ("peace in our time") and twists it in a very Asimov sort of way to mean all of human life must end because there will be no peace with people. He overcomes Tony's personal digital assistant (Jarvis), inhabits a robotic body and uploads himself to the Internet so he can escape into anything he wishes.
There's a fight, Ultron escapes, he finds some vibranium (helpfully expanding the MCU slightly in the process with references to Wakanda), and escapes again, this time with the help of the two enhanced. He escapes to South Korea where he uses a scientist to create a "real" body for him (how this is appreciably better than his upgraded body he gets later isn't really explained). Through another action sequence, the Avengers steal this body and then bring it to life so that it becomes Vision. Everything converges in the invented Central European country Sokovia (where the movie opened) where Ultron, who's been talking positively of the asteroid that destroyed the dinosaurs throughout the movie, has creating a system that will life the capitol city up in the air before dropping it to the ground with enough force to cause a global extinction event, which the Avengers avert. Because of course.
So, there's the plot. A lot is going on. I think the stuff around the creation of the body that becomes Vision is really just there so we can get Vision. As part of Ultron's plan, it feels extraneous at best. His end goal is human extinction. Having some sort of humanish metal body feels just extra. But around this large, somewhat unfocused, plot we have our heroes, and as I implied earlier, not a lot of them seem to be there for any other reason than they are Avengers.
First, let's talk about what this movie is supposed to be about. The first Avengers was really an excuse to get superheroes together to fight a heavy. The underlying idea was about team building. It was facile, but it worked and every character was functioning towards that goal. In this, the goal seems to be centered around Tony Stark's desire to create a replacement for the Avengers in the form of a highly intelligent AI that will protect Earth from threats originating from off world. It's an extension of where Tony was left at the end of Iron Man 3. He wants Iron Man to be unnecessary. The creation he ends up with fights back (using "I've got no strings" from Pinocchio quite effectively in a creepy sort of way), and Captain America's assertion that humans should always be part of the decision making process ends up correct. In terms of characters who seem involved in this thematic idea you have Tony, Bruce Banner, Captain America, and Hawkeye. The rest feel extra, taking the movie in different directions.
Thor is the one who seems the most out of place. He's there for a literal reason (Loki's scepter is supposed to go back to Asgard with him), but outside of that he doesn't really contribute to any of the movie's core idea. In fact, he famously goes off in the middle of the movie to help inform the audience about Infinity Stones. They sacrificed cohesion of the film itself in order to help sell later installments of the franchise, and the movie suffers for it. Black Widow also feels like she's in another movie. The exploration of her past is interesting, but I don't see how it feeds into the ideas at the movie's core. What this does overall is create a disjointed experience that keeps the movie from coming together as a single work.
There's quite a few individual pieces of the film that I like quite a bit. The action scenes are fun (though, we get another big CGI army). I really like Ultron himself. He's not just a quippy form of Loki. He's driven by an ideology and a set of beliefs all while talking in a way that betrays the malfunction at his core. The relationship that begins to blossom between Natasha and Bruce is both sweet and tied to some interesting unique parts of their characters (Natasha's insistence that she's a monster and her ability to draw Bruce out of the Hulk). And I really like the central idea and how that manifests in Ultron. I just wish the movie came together a bit better.