This post includes major spoilers for Avengers: Infinity War
It had no business being a good movie. And yet it was. How did that happen?

I’m talking, of course, about Avengers: Infinity War. My rule of thumb is that, all things being equal, the quality of a movie is inversely proportional to the number of major characters. How many were in this movie? I won’t even try to count.
So why does it work? The key is to listen to the directors’ commentary. In other franchises (let’s leave them anonymous, shall we?), what excites the director is the special effects.
This scene was shot against a green screen / CGI. So was this. And this. Oh, that scene? It was filmed on set. No biggie. But THIS scene, that was green screen / CGI. And so on.
Not surprisingly, while that (anonymous) movie has whiz-bang action and CGI, it isn’t very good. Story and character were pretty much completely overlooked.
In the commentary for Infinity War, what the Russo brothers comment on is story and character. And oddly enough, when the emphasis is on those two things, the end result is pretty good. This is the thing about Infinity War. While I’d argue it wasn’t a great movie, there was greatness in managing to bring all those characters together in a coherent, character-driven story that gave many of the characters interesting development arcs.
The most obvious arc is Thor’s, who picks himself up from his defeat at the hands of Thanos and, by the time he arrives on Earth, has become the king he was meant to be. Groot has a small but satisfying arc in which, after witnessing the sacrifices made by Thor, grows out of his annoying teenage phase and becomes instrumental to Thor’s evolution.
The humour, and there’s plenty of it, is character-based. Peter Parker, with his love of “old” sci-fi movies. Poor clueless Drax who thinks that by being still he becomes invisible. The clash of egos between Tony Stark and Stephen Strange. Then, of course, there’s the culmination of ten years of the MCU, the moment when Rocket meets the Winter Soldier and says, “I’m going to get that arm.”
There are many other precious moments. Pretty much all the scenes with Vision and Wanda, with their new (and apparently, doomed) affection for each other. The reverence with which Peter holds Tony Stark. The moment when Steve Rogers and Thor compare haircuts and beards in the midst of a battle for the universe. When Bruce Banner and Natasha are reunited, scarcely a word is spoken. There’s no need, because their faces say it all.
In the end, it’s the humanity in Infinity War that makes it good, and the reason why we so look forward to End Game.