Since wingzero0 got the initial discussion of Infinity War going. Here are the thoughts I had the morning after I saw it:
This movie worked for me for two big reasons:
1. Making Thanos the protagonist was absolutely brilliant and, in retrospect, was probably the only way to do this movie with so many characters and moving parts.
2. Using Thanos’ iconic "snap" to essentially ‘erase’ most of the new characters we’ve acquired since the first Avengers movie is a brilliant way to clean the slate prior to the next Avengers movie, so that we can return the focus to those core characters for what is quite-likely many of their swan songs and the logical close of their narrative arcs. If Cap or Iron-Man had died in this movie with so many characters, their sacrifices will be somewhat lost. If and when the die in the fourth movie - which will benefit from fewer characters and thus greater focus on those who remain - the impact will be greater. Even more so as they likely die ushering in the return of those characters who will be the focus for the next major phase of the MCU.
As a bonus, "killing" so many people, but none of the original core Avengers, was an absolutely brilliant subversion of audience expectations.
Obviously all the small side stuff is also great; the clever pairings of characters you wouldn’t expect, the callbacks, the perfect end to Loki's arc from Ragnarok, the quips, etc. But it’s the two above points that really elevate it, I think. Of course, if the follow-up movie doesn’t deliver on what point #2 set up, I reserve the right to revise any of the above opinions