A mixed bag

Rating: 6/10 (9 August 2019)

*spoilers*
This final massive movie in the massive MCU franchise reaches an end to the massive buildup after 22 movies, and is also the continuation/ending of the Infinity War movie. Obviously, the makers of this movie were given carte blanche to do whatever they like to the superheroes, since a few were killed off or forever changed in ways that cannot be recovered. I'll leave the plot to the Plot Summary section and review the good and bad elements of the movie (as opposed to the many overly-emotional reviews and votes here that see only black or white):
Let's start with the time-travel, since most people seem to be having problems with it and it is the major plot point by which they try to fix what Thanos did in Infinity War. In fact, the writers did an interesting job making use of time-travel without it feeling like a cheat. They simply changed the rules of time-travel used in other movies in order to avoid the usual time paradoxes. This may feel like a cheat to some, but, obviously, nobody can claim to know how time travel could work. And if you pay attention, they stick to their own rules consistently, which is good.
Here is the explanation for those that missed it: Timelines can be changed, but when they are changed, they split only for the people involved. I.e. You cannot go back in time and 'fix' the timeline for other people because those people would remain in that alternate timeline that you left behind. You can, however, change the timeline for yourself, and the past/future all proceed in the order by which you experience/d it. You can also change the timeline for the people in the past, but, once again, that would not affect the timeline that you left behind in the future.
So, to make this clearer with examples, there are at least three timelines for the people in this movie: 1. The one we know according to the superheroes where Thanos wiped out 50% of all life. 2. The timeline for Thanos himself (and his friends) that has now been changed when he jumped forward in time. He didn't wipe out 50% in his timeline and he changed his mind and wanted to wipe out ALL of life this time after he saw how ineffective it was in one possible future. 3. The alternate timeline by the people in the past after Thanos jumped forward. Their timeline split and they experienced no Thanos!
There is no problem with changing timelines. The only problem is that they needed to fix their own (#1) for the rest of the world, and going back in time would not do that. They needed to bring something from the past (the stones) in order fix the present timeline for everyone. The only reason Captain America had to go back to return the stones was so that alternate timeline #3 would not be wiped out without the stones. While he was there, he stayed there, which basically created his own personal alternate timeline that followed #1 with #3. All clear now?
This new method of timelines achieves something important: It doesn't cheat by going back in time and undoing/changing it. It forces them to be creative and to do something in THIS timeline in order to fix the problem. Thus avoiding the typical cheat of time-travel.
Now for the rest:
Killing off Black Widow, Iron Man and Captain America (technically he wasn't killed but he lived his whole life elsewhere so he can't continue here and now in a sequel in this timeline so it's the same thing..): This was actually very good and brave. I wish they had done this sooner, then the superheros would actually feel more vulnerable and human. Iron Man gets a strong ending to his character arc and a touching send-off. Cap also gets strong character development here.
The mixing of so many superheroes in one movie is bad for several reasons. First of all, it's like mixing fifty of your favorite foods together. They work nicely alone, but together, it becomes an overblown mess that doesn't work. Second, not all superheroes are fun for everyone. So mixing foods you do not like together with the good ones, will simply ruin the whole salad. Third, even with three hours, many of them are neglected.
Hulk is ruined. Someone very misguided thought it would be clever to mix Banner and Hulk into one person. Except that the whole fun of Hulk was his uncontrolled fury and his split personality.
Thanos became too powerful in this movie. I agree with this complaint. This is inconsistent with what we saw earlier.
Captain Marvel has got to be the worst character by far in the MCU. Her personality has all the charm of an angry gnat, her obnoxious feminist attitude is toxic, and her super-powers are so badly defined, they can do anything and everything as needed. Unlimited and/or undefined super-powers are boring and are guaranteed to ruin any super-hero movie (which is why I also dislike Superman). Her presence in this movie almost ruins it completely but thankfully they used her as little as possible (probably because they realized how ridiculous her powers are). Most of the stars I removed were because of this character.
They have increasingly made Thor into the point of ridiculousness in a series of movies. And in this movie he has reached a peak of silly uselessness. It is kinda funny though so I don't hate it. But Thor fans will not be pleased.
The final fight is a massive spectacle on one end, but so overblown and full of CGI it makes me yearn for the simpler superhero movies. The inclusion of the whole universe simply isn't as relatable as a local problem in one city. It's just too much. I enjoyed earlier simpler super-hero movies much more than this one.
In summary: A mixed bag. It's good that they finally put an end to this overblown series of movies that have gone out of control. It's not anything close to a classic in my book, but it's difficult to ignore since it concludes the series and it does have lots of fun stuff despite its flaws.