As others have said, that first act is a real slow burn. Showing the gravity of what they lost in the snap through Scott Lang was inspired. The world as a whole feels legitimately broken by what happened, as it should. That five year jump blindsided me completely, but I really like how it's not reversed in the end either. it's not like they go back and murder Thanos before he gets the stones and then the world changes like it never happened. Five years is a LONG ASS TIME. That makes what happened in Infinity War still substantially consequential within the universe. It's something that's going to have to be reckoned with in all the direct sequels of characters that were gone for five entire years. What's Wakanda going to look like? Though I gotta say, it's going to be awfully coincidental that every one of Peter's core quiz bowl team were snapped and came back five years later the same age.
The second act is a little campy but all in good fun. The victory lap of revisiting previous movies is a fun narrative device without feeling cheap or self indulgent. However, time travel brings with it so many other headaches. They say explicitly that going to the past and changing it will change your future, but they not only kill Thanos and his entire army, they kill a time displaced Nebula, bring a time displaced Gamora to the future, and lose track of Loki and the tesseract. None of those discrepancies are ever really addressed and that's pretty annoying in a film that takes great pain to explain why they have to return the stones to precisely when they take them from history.
The second act also introduces merged Hulk, aka "The Professor." I don't mind this progression in the character in principle, but the sudden change is one of my least favorite things about this film. It really feels like the Russos glossing over Bruce's character development over the preceding 5 years and then never doing anything with it. Bruce does not get an arc in this film and that's a damn shame, especially when there was more than enough set up for one. Hulk never gets a redemptive moment or one final showstopping display of power. Out of all the central Avengers, he felt the most short changed. Ruffalo had hinted multiple times that they were going to do an extended story for Hulk between Thor Ragnarok, Infinity War, and Endgame, but all the formative change for him occurs off screen during the time jump. Savage Hulk should have made a reappearance at the end with Hulk having one final display of astonishing anger. We don't know if we're getting Hulk back in any sort of meaningful role, especially since his right arm is still seen as severely injured in the aftermath scenes of the film.
Dude-Thor is introduced in the second act as well, which was a clever and subversive turn for the character that was probably a little too played out by the end. Thor's arc suffers similar problems with Banner's where a big substantive change happens off screen but at least the story goes somewhere with it when Thor visits Asgard and reconnects with his mother. The decision to have thicc Thor for the entire movie was also a...bold choice I wouldn't have ever predicted. Thor staying the way he is, at least physically, is a little more palatable than Banner's staleness because at least he has greater potential for a follow up. Thor joining the Guardians runs the risk of overshadowing them, so I expect he will be written out of the group somehow before or during GotG v3, but at least he embarks on another adventure and change is expected. Thor's character has also established a somewhat cyclical pattern of destruction and rebirth, so the regression to a psychologically broken and fat Thor is in line with the character. I'm torn on whether I wanted him to have a bigger redemption arc and will likely need reviewing to really make up my mind.
One other thing I wanted to make sure I touch on with the second act is that with the reintroduction of Thanos from the past, I've noticed some complaints that this Thanos feels like a watered down version of what we got in Infinity War. While I understand the sentiment, I think watching Infinity War and Endgame back to back will help mitigate that perception. Thanos in Infinity War was the protagonist but that version had won at the point of his death. And as I said earlier, with 5 years time passing, he's going to leave one massive scar of trauma on the universe. The past version of Thanos is no longer a real character in Endgame and I think that's fine because as the primary threat, the time displaced version of Thanos is merely a promise of what's to come. Both the past and present versions of the character refer to himself as "inevitable." That promise is his sole purpose in the film, and with the level of introspection presented in the preceding movie, I don't think Endgame really needed to unearth more layers for Thanos.
The second act ends with Natasha and Clint fighting over who gets to sacrifice themselves to obtain the soul stone which was handled pretty well overall. Their entire dynamic in this film feels like a wonderful callback to the first Avengers, and their friendship feels authentic throughout. I've come across some criticism of Natasha's death saying it felt flat in the context of the film and considering how significant of a moment it should have been, and I agree that it doesn't come close to Gamora's death in Infinity War which it's trying to emulate both in tone as well as narrative significance, but that mainly comes down to the lack of follow up after it happens. After Gamora dies, the event continues to have aftereffects with Quill's reaction to the news on Titan and Thanos having a vision of her within the soul stone when he does the snap, but Natasha isn't really referenced again in any meaningful way after she's killed off. In that sense, they really short changed Black Widow as a character. Either Bruce or Steve should have been hit much harder than they were by the loss.
Moving on to the final act, what's there to really say? The final battle is everything I could have asked for. Captain Marvel's entrance is clearly meant to be emulating Thor's entrance to Wakanda in Infinity War and doesn't quite live up to it since Carol doesn't go through an entire arc beforehand, but it was still a major crowd pleasing moment. That's where I think Bruce could have really used a moment to shine and bring his character back around to embracing the savage angry Hulk. The moment that most closely resembles Thor's IW entrance would have to be Cap wielding Mjolnir which was so over the top fan service, I don't even care that it was fan service. That entire sequence and his use of the hammer + shield combo was outstanding. Tony's death and final sacrifice was seen coming a mile away but still hit home really hard. You could hear the entire theater holding their breath as his life slowly slipped away, and the funeral scene was a touching send off for the character (that really should have included Natasha as well but whatever, opportunity squandered). Steve and Clint having happy endings was also a great narrative choice as it helps emphasize the underlying message of these films. Tony in Age of Ultron has a great moment when he asks "Isn't that why we fight, so we get to go home?" Steve deserves his happy ending, and so does Clint, because not only does it help give the entire preceding saga an underlying sense of optimism, robbing either of that kind of closure would feel pretty cruel at this point. At least Tony got to experience fatherhood and redeemed himself by successfully bringing back his surrogate son. A lot of people wanted Steve to sacrifice himself in the end, but not only has the character already played that exact same move before, how much does one man really have to suffer before he finds a semblance of happiness? It's a fitting end for the character, though I wonder whether or not that ruins the timeline in some way. Time travel is such a clusterfuck.
As much as I loved it, what ultimately keeps Endgame from matching Infinity War for me is a lack of narrative continuity, and much of that has to do with Thanos being killed off in the first few minutes. As I said, that Thanos won, and his narrative thread ends with his life, but his philosophy should have continued to play a role in the story. That it was never really raised again or criticized directly felt clumsy. Endgame feels like the natural endcap to Infinity war, like a long protracted final act, but without the core narrative thread you'd expect to connect the two films. It's not even that they had to jettison those questions to proceed with the plot. There were plenty of moments they could have weaved it in using the Thanos from the past but instead chose not to. This makes Infinity War and Endgame feel like completely isolated films which is in conflict with pretty much everything else that feels like a direct continuation. To put it simply, Thanos had his character arc come to a complete close at the end of Infinity War, and Endgame chooses not to pick up that old thread. I've only seen it once, so I hope this sentiment changes for me on repeat viewings. This lack of fluid continuity between the two films along with the complete bungling of Hulk's story are what irks me most about Endgame. Both are handled very inelegantly, as are some aspects of time travel, which feels like a step down from Infinity War's immaculately constructed narrative.
That all said, as a capstone and send off for the original incarnation of the Avengers? Damn, does it deliver. The film is a tribute, the marvelous Stan Lee cameo not withstanding. I'm excited to see it again as soon as possible