I actually thought they did a pretty tremendous job fleshing out characters and providing character arcs and beats for most of the key players. They gave Thanos an at least passable motivation to serve as a sort of moral dilemma. Kept hitting father/mentor/legacy beats with Tony Stark and his relationship to Peter Parker coinciding with his struggles or burden of leadership and sacrifice. Even the alpha male competition dichotomy (and later trichotomy) with Stark/Strange/Star Lord was entertaining, mostly because the interactions and the plot progression were centered in each character. There were decent juxtapositions of the large stakes of Thanos wiping out half the universe with the smaller stakes of things like Scarlet Witch trying to save the mandroid she loves.
I could go on and on, but at the end of the day, this is much more like jumping into Season 3 Episode 16 of a non-serialized TV series more than it is jumping into a movie. Its biggest strength is also it's biggest weakness, which is that the movie doesn't need to spend time on certain foundational work because 18 films before it already have. I thought the Russo brothers did a mostly great job, though.