(Reactions not written as they happened, but arranged on the timeline as if they were.)
Okay this was a good start to the three hour capstone of their franchise. They kill God in the first five minutes - which is both to say the big purple guy who acted as Authority, and also any hope of using the old methods to return things to harmony. All that's left is the bleak freedom/existence of moving on in the world under your own power.
Five years later, the reemergence of Antman brings the fuel of innocence to our heroes. They've given up, but he's as angry and hopeful as they were on Day 1 of the postSnap world, and that kickstarts them into action. And the initial sheer craziness of his idea "let's invent time travel" also fits this existential theme. Let's just do whatever the fuck we want, nothing is impossible, we can fuck with the nature of the cosmos if we want to.
Unfortunately this is where the movie starts... going back on the rails, and becomes worse over time. Their solution of course is to "steal back the past." This is not just recovering those elemental stones which were destroyed, but also represented by diving back into previous movies of the franchise. The solution to our problems is retrieving the Way Things Were in Avengers 1 and Guardians to the Galaxy and Thor 2 (???)
Also a sidenote: the Infinity Stones themselves - aka the Captain Planet elemental harmony of this universe - are just criminally under-explored. Like yes we have two good scenes about the Soul Stone, but all six of them should be doing something different and interesting and powerful, that makes their combination more interesting than "you have collected 5 out of 6 Macguffins." What is wielding the Mind Stone like? How does using the Space Stone change you? What is the Reality Stone even doing? (Granted Doc Strange did discuss the Time Stone some, but let's just admit that was bad even if explored.) This would be whinily asking for a fictional universe to reveal more of its wikipedia to you except that each stone got at least a whole movie about it. They spent a movie on each stone before uniting them, and we still know barely anything about them other than "with our powers combined." Criminal.
Anyway, having begged the past to return, we also get the return of Thanos. And from there the movie goes downhill. He's not even the principled Authentic Evil of the previous movie (and the beginning of this one) who is noble to others in his insanity. No he's just an abusive asshole now, who takes grim delight in the pain of others. This Thanos does not pick his own gourds. And now we have to fight him, again. A real fight with both sides at their full power this time.
When the heroes return to the present and re-summon the dead half of humanity what we immediately see is... a Hell World. Nothing but blasted landscape and catastrophe. (This is just because of the missiles from Thanos' ship, but our perspective is so confined to this that it represents the results of their transformation.) This is the result of their hubris. (And the cavalry arriving is portrayed as portals from glowing, heavenly worlds.)
The gauntlet. They have to emphasize flabby Thor for two hours just to bypass why he couldn't use the gauntlet. He's a god, lightning incarnate, he is the obvious candidate for it (other than Vision, another sadly underused element - created to be some sort of android Messiah, and never used to fulfill that.) But this means that true power can be wielded by the Hulk - in this movie a new age New Man who has found balance between his emotional and rational halves, and so uses the gauntlet to restore Just The Way Things Were but also Not Losing Anything We've Gained Since Then.
The actual question begged of the gauntlet, of the entire franchise since Iron Man 1, is Tony Stark wielding the Infinite. This would actually confront his thirst for control in well, the way that comic books imagine these things (and he makes a much, much better villain than Thanos 2 Electric Boogaloo.)
And so we finally get a shot of Iron Man picking up the symbol of ultimate power, using it to destroy his enemies and Make Everything Right. He starts off by immediately genociding Thanos' slave army. Alright this is the corruption of power narrative I can... oh he's dying. That's just it, the end of the story. He goes out a good guy, having made the ultimate sacrifice, to kill a few thousand or million faceless aliens. Not anything actually good, but also not anything we'd care about so we can properly understand what was wrong about Stark Industries/Hydra all along.
Sidenote the second: Endgame is a chess term for the stage of a chess game where there are few remaining pieces, and so you can just brute force calculate every action, instead of having to rely on heuristics and patterns and guesses. Just two kings and two queens, etc.
I'm not one to fault the fan service superduper explosion combination of every superhero we've met over two dozen movies, but it's ironic how this overpopulation was the exact opposite of what the term means.
ADDENDUMS: Wow Karen Gillian got her payoff finally, after being very underused in her previous movies. I guess the writers just felt what the audience really wanted was lots of Nebula. It's out of nowhere, but I don't really disapprove, given that her character role is "to be the lolfail everyone kicks for not being as cool as them" (Thanos, her sister, the crew) and they just really rubbed that in, adding "her other timeline self" to the bullies that dunk on her. In some ways she is the ultimate Hufflepuff character, and I'm fine with her being the center of the capstone movie then, it's like Neville Longbottom actually becoming the main character of Book 7.
And I can think of nothing that is more MCU than taking an actually interesting scene that used music and imagery to draw people into a movie that was considered questionable - Starlord dancing by himself in the opening credits of Guardians of the Galaxy - and convinced audiences that "yeah 80's nostalgia plus alien landscapes is kinda cool and we are happy to watch even if it doesn't have Nick Fury", and returning to view that oh-so-slight-numinousness from the outside and just saying "wow he looks like an idiot." Keep being you, Marvel.
