Spoilers for Avengers: Infinity War (2018), The Avengers (2012), Back to The Future 1 & 2 (1985 and 1989 respectively), Halloween (1978)!
To my mind there are three movie types: Tentpole, standalone, and bridge. A tentpole movie is usually a major part of a franchise. For example the movies Iron Man (2008) and The Avengers (2012) are tentpoles that helped form the Marvel Cinematic Universe. Without tentpole movies a lot of franchises tend to become stale. In other words, no disappearing Michael Myers at the end of Halloween equals nothing important happening to set the tone of a franchise.
In the case of Avengers: Infinity War one could argue that it is a tentpole. Yet I would disagree, since the only point that would make it a tentpole is the way in which it ends differently in comprison to other Marvel Cinematic Universe movies.
Most movies are standalone movies, including those that are part of a franchise. Though franchise movies like Back To The Future 2 will often not work as well for viewers without some knowledge of the original source. In the case of Back To The Future 2, it is knowing what Marty McFly does the first time around that makes the movie work.
While, in the case of Avengers: Infinity War, it is knowing things like what Tony Stark (Iron Man) is talking about when he mentions seeing the threat of Thanos coming for several years. Personally, I believe Avengers: Infinity War could almost be considered a standalone.
However, it feels more like a middle act based on a bit of dialogue about an “end game.” (Well, that, and how the comic book series Infinity Gauntlet (July to December 1991) gets resolved.)
Since it is a middle act, like Star Wars: Episode V: The Empire Strikes Back is for the original Star Wars trilogy, we can instead define it as a bridge movie. A bridge movie is one that connects one part of a franchise to another. In this case Avengers: Infinity War bridges the gap between The Avengers and Avengers 4 (2019).
In conclusion, reviewers and viewers that are slamming this movie for various perceived faults may not have considered exactly what movie type Avengers: Infinity War is and incorporated that into their criticism.