Spoiler (click to reveal)
Leaving aside the fact that Thanos' plan makes no sense, I really didn't love the way they handled him. Maybe it's a little silly, but I really like the whole 'in love with the personification of Death' thing. Though I suppose that would have required them to get into a lot of the weird cosmology stuff that they probably didn't want to, not least of which because the vision seems to be to keep things as grounded as possible.
This also extends to the Asgardians. It seemed like some steps in the right direction were made in Ragnarok, but here we've gone mostly backward here. Even their ships are just grey metal cubes. Even in the first Thor the Asgardian ships, armor, and architecture were allowed to have an interesting and distinct style.
But then the infinity stones, and the soul stone/Red Skull stuff specifically is exactly the kind of mystical/spiritual that I wish Marvel felt more comfortable doing more consistently but they seem to avoid as much as possible. So I liked that but was also surprised that it was allowed in.
Then there are the issues that come up with any team-up scenario of enforced power parity. The Hulk/Thor with his new super hammer types are equally efficient at killing mooks as Cap, Black Widow, and Bucky, and I think it was even the case that there were members of the Black Order that were roughly equally good at fighting members of the first and second group. The most ridiculous thing was how Cap and Bucky were first to show up to hold off Thanos right at the end rather than Thor, who once he finally did show up basically took out Thanos in one hit. I get that it needed to happen for the plot to progress, but it made no sense for the characters to behave that way.
Also, the impact of everyone dissolving was pretty minimal, it's clear that the remaining heroes (who, coincidentally, are basically the original core Avengers plus a few extras) will just have to team up to get the gauntlet and un-dissolve everyone in the next movie. The only real question is which of them will have to sacrifice themself to do so. My guess is RDJ, because he seems like he's ready to be done with these movies, he's probably becoming too expensive, he was weirdly bothered by Thanos this whole movie, and he's clearly necessary to the single way that Dr. Strange saw the good guys winning or else he wouldn't have exchanged the time gem for Tony's life. Plus it would be nice for these movies to get out from under his shadow. But I could also see it being Cap, he can't be cheap either and he is the most conventionally heroic.