A great first two hours and a meh final hour

Rating: 7/10 (26 November 2019)

This is more built like a traditional movie with a recognizable three act structure than Avengers: Infinity War was. I quite like it overall, but that traditionalism requires an antagonist that isn't really needed, I don't think.
The three acts are pretty distinct. The first is almost purely character driven. Our massive roster of heroes has been culled to almost exclusively the original six (Cap, Thor, Iron Man, Hulk, Black Widow, and Hawkeye) with a few others (Rocket, Nebula, Ant-Man). Considering the film's length, this gives each character enough time to stand out. I think the decision to jump ahead five years was very smart. The characters have all changed in those five years, and we have to spend time with each of them getting to know them again. We're essentially being introduced to new characters, and the movie takes its time with it. My favorite of these, not based on the introduction but based on what comes later, is Thor.
Thor is consumed by his failure to aim just a bit higher when he threw his axe at Thanos. If he had hit Thanos in the head instead of the chest, Thanos would not have performed the snap, wiping out half of existence, so he sits in a small corner of Norway where the remnants of Asgard have established a new home. He plays video games, drinks copious amounts of beer, and yells at 12 year olds who make fun of the rock monster Korg online. He's a sad, fat, king who lords over the small domain he wishes to keep (his living room). We see the other characters in similarly different states. Tony has walked away from a life surrounded by technology and lives in a nice wooden cabin by a lake. Cap leads self-help meetings. Hulk has found equilibrium with Bruce Banner. Black Widow stays in her office, sending other people out to search for Hawkeye who's left home after his family disappeared, killing those he deems worthy of death.
It's a surprisingly somber and solid hour of character based storytelling, introducing us to the new rules of a new world with familiar characters who've all changed. The one who hasn't changed is Ant-Man. Scott Lang was trapped in a gobbledygook science state that he thought only lasted five hours but actually lasted five years. He hasn't changed, and it is through his eyes that we mainly see how things have changed. He's also the impetus for the movement that leads to the second act: The Time Heist.
I've seen the Time Heist described as fan service, and I can't entirely disagree but I do in part. There's so much more to this than just seeing things from the past that Marvel fans liked from slightly different angles. That's definitely part of it, and we get some standardly amusing quippy humor, especially from Ant-Man, Iron Man, and Cap ("America's ass" indeed). However, there's a lot more going on especially around Thor. Thor goes back in time (using gobbledygook science that Tony Stark pulls out of nowhere because movie) to Asgard during the events of Thor: The Dark World. The task of retrieving the Beauty Stone becomes completely secondary to Thor's time spent with his mother in a wonderfully touching scene between mother and son where she helps him find his purpose again. It's the sort of emotionally driven scene that believably takes a character from one place and gets them into another. I mean, a tender visit with your dead mom might do just that.
Before going any further, you have to talk about this new (old) version of Thanos. Thanos gets his head chopped off in the first sustained sequence of the film, but with time jumping, the older version of the heavy gets introduced. I have to imagine that there's a very early draft of this film that doesn't include Thanos at all past the fifteenth page because Thanos has nothing to do with the central conflict of the story (jumping through time to collect the Infinity Stones). He's purely extra. He shows up after the task is done to fight the world's mightiest heroes and introduce a late, unrelated, obstacle.
And that's what the last hour is, an unrelated fight that breaks out in epic style and is just packed full of fan service. The last hour is little beyond that. Every hero comes back when Hulk reverses the snap, and we get an even more concentrated and bigger version of the fight as in the last film. Don't get me wrong, this stuff is fun to a certain degree and well assembled, but the story's effectively over by that point and yet it just keeps going. I loved the movie up to this point, and then I just kind of got deflated. "Oh, this is what we're doing now, huh?"
So, yeah, I loved about two hours of this and tolerated the third. The first two hours aren't perfect (Tony thinking about time travel for five minutes and finding the perfect answer is just the kind of ludicrous that Marvel is permeated with and sort of just bugs me), but the strong stuff there is very strong. It's just kind of too bad that the movie "needed" an antagonist shoe-horned in its final act in order to make fans happy.