My thoughts mirror Galan's for the most part, with a few exceptions.
Spoiler:
Although we know time-travel isn't actually a real thing, and there is no "right way" of doing it, it was still not explained very well in the movie. Luckily, they didn't spend too much time on it, and it was used as a vehicle for giving us some really great scenes, especially those in Avengers tower during the events of the first Avengers film. Also, if you don't like that they used time-travel... Did you not watch Infinity War? It was foreshadowed heavily.The Widow death scene was executed kind of poorly, imo. It lacked a kind of poignancy that Stark's scene also lacked, but to a larger extent. If Jeremy Renner wasn't so damned good, the scene would have been much worse, I think.
To all the people that don't like how Thor was portrayed? You brought this on yourselves if you celebrated Ragnarok. That movie started "funny Thor", and though I don't agree that he was made to look weak or anything (even fat and lazy, he was still more powerful than most), it's a pattern that's going to continue in to future movies, and to a larger extent.
Hulk still hasn't been "fixed", but he's getting his own show, and hopefully that will be a way for them to re-establish him as the force of nature they portrayed him as in the earlier movies. I will say this, though: That CGI was insanely good. Big, green Mark Ruffalo as Professor Hulk? Someone needs to buy those animators a drink.
The scene with the Ancient One, I really liked. Her realization when Banner tells her that Strange just gave up the stone was played brilliantly, and was a strong character moment which, incidentally, is what this movie does best.
All the best things about this movie are the character moments, and it's obviously the Russo's strength. Tony, Hawkeye, Scott Lang's frankly surprising huge role in the movie were all achieved incredibly well, but one stands far and above the rest: Steve Rogers.
I've said on more than one occasion that he's the best thing about the MCU; equaling or even surpassing RDJ at times with how good of a portrayal he's had by Chris Evans. And, being the fan of The First Avenger that I am, it was still fresh in my mind sitting down to watch this. It is the single-most perfect ending to Captain America's story that they could have done, and I think in years to come that people are going to, or at least they should, remember that it was Cap and Tony, not just Tony, that made the initial arc of the MCU as great as it was.
The only thing he didn't get to do, that I would have liked, was give Thanos his "as long as one man still stands" speech. If he'd done that just before the reinforcements turned up, it would have probably been my favourite MCU moment ever.
Speaking of which, that part where they all come through the portals? Brilliantly done, even more-so than Thor showing up in Wakanda in the previous movie. The entire battle scene with Thanos and his army is incredibly well-done, if a little dark cinematography wise.
The whole "girl charge" thing didn't bother me as much in the context of the movie. It was as subtle and as cringe-worthy as I've come to expect from anyone working for Disney, and it makes me believe that they don't really care about treating male and female characters the same. They're disingenuous, they always have been.
I do think they can do good things with Captain Marvel, though. I hope they do. Regardless of how much I might think Brie Larson doesn't give a good account of herself, it would be hypocritical to punish CM for that and not, say, Iron Man, given RDJ's history as a human being. They have work to do though, especially after dropping the ball with the likes of the Scarlet Witch.
X-Men is on the horizon, and for those that don't know it, X-Men has probably the best roster of women characters in all of comics, and an X-Men MCU movie has the potential to take what Wonder Woman did for female superheroes on screen and launch it to new heights... If it's done properly.
TLDR: The movie isn't perfect. It has plot holes. It doesn't work in a vacuum as a movie at all, but then again, it's not supposed to. It's the final issue in a comic book storyline that we've been reading for over a decade. Your enjoyment of it is going to depend on how much you care about certain characters, or the Avengers in general. I think in a lot of ways, Infinity War and Endgame have become the "Star Wars" for this generation, and when they reboot it in twenty-odd years, you'll have people who are teens now lamenting that the new ones aren't as good as the originals.