I'll try not to spoil anything here, so here goes...
In 2008, Iron Man kicked off this "Marvel Cinematic Universe" aka the MCU, and it's gone in phases. Some of this was natural, some of this was modified when Disney would acquire the rights to certain characters down the line, like Spider-Man most notably. Shame he couldn't have been there since the beginning.
Most of these movies have been pretty fun over the years. I still haven't seen Black Panther, but I wasn't lost. I guess that means his story was rather throwaway, but I couldn't say.
So this movie begins right about when Thor Ragnarok ends, and Thanos kicks ass and takes names. He shows us he isn't fucking around, because he wants those stones. He'll come to earth to get 'em, and that's where these characters start coming together. Provided, there's never one ultimate fight with everybody, as they're divided in groups for the most part:
Iron Man, Spider Man, Dr. Strange
Thor, Guardians of the Galaxy
Captain America, War Machine, Black Panther, Bucky,
Vision, Scarlet Witch
Seem a little empty to you? Yeah, that's right. Nowhere in this movie will you find Ant-Man or Hawkeye. They just ain't there, for some odd reason.
Bucky is pretty pointless, as is War Machine (like always), Black Widow, and Black Panther. Hulk is barely in the movie, because Bruce Banner had a hard time getting angry. Hulk was wimping out after Thanos kicked his ass, and wouldn't come out to help anyone. There's a scene with Hulk running with Captain America, Bucky, and Black Panther that isn't in the movie. Hulk is barely in the movie, and when he is, it's mostly Bruce Banner in the Hulkbuster suit. Kind of a jip, but okay.
Otherwise, everyone did great with their material. I think Vision is a stupid character, so his scenes and Scarlet Witch's were pretty meh to me. Otherwise, this movie didn't have any room for filler. There are a lot of fights and mindless action scenes, and Josh Brolin is more ominous as Thanos than what's-his-face as Apocalypse in X-Men. Thanos is actually a be-all-end-all villain, for a change, and if there's one reason for any of you fans of grim cinema to watch, it's because the ending of this movie is really bleak. I love it.