It's hard to say too much about this movie without spoilers so this might be short and vague.
-One you need to know: This movie isn't a full story. If you remember, early on the plan was to film Avengers 3 and 4 together. When it was decided not to do that Marvel emphasized the idea that this wasn't necessarily one continuous story. That may have done a disservice to fan expectations, though it's probably technically true. This movie is very much like a jaw-dropping season finale that ends with a cliffhanger and leaves you desperate for the next season to start, so set your expectations appropriately.
-I've seen reviewers write that this movie is pretty demanding of fans as far as expecting you to know things. That's reasonably true. If you haven't seen Ragnarok, Guardians of the Galaxy Vol. 2, Civil War, and Black Panther you're probably going to be slightly confused along the way.
-This movie should be called Thanos because he owns it, he's basically the protagonist and gets the most screen time. The CGI is really impressive and makes him into a character, not an effect. Josh Brolin said the other day when someone asked him which character he preferred playing, Thanos or Cable, he said Thanos because he was so amazed that you could see his performance in this hulking purple monster. It's amazing, and well worth the years of build up. As hard as it is to believe he's almost sympathetic. Some people complain about Marvel villains, they won't get any more ammunition from this movie.
-The interactions between the different sets of characters is a lot of fun and everything I hoped for. The only drawback is there's not more of it. The characters are also very consistent with their past appearances, especially the Guardians who feel like they stepped out of their own movie.
-The movie has a 2 hour and 40 min runtime and it flies by. That does mean you may want to take it easy on the soda though.
-There were a lot of young children in the showing I went to. While this isn't any more violent than other Marvel movies it feels more intense. I definitely wouldn't bring kids.
-That said, the movie is still very funny at times. There were a few lines that had the audience laughing so hard I couldn't hear the next line of dialogue.
-There are also more emotional scenes than in any other Marvel movie I can think of. There's one in particular between Thor and Rocket that brought everyone to tears and is the best thing Chris Hemsworth has ever done, it also makes up for the humor undercutting the drama in Ragnarok.
-There's no mid-credit scene and one post-credit scene. It does help set up the next movie a little, but unless you're reasonably familiar with the comics it won't mean much to you. So, basically, if you feel like you need to get out of the theater, go ahead because you might not understand it anyway.
-As usual, this will require a rewatch or two to determine my final ranking but it's hard to imagine putting it below any other Marvel movie, but it's such an achievement it's difficult to think of it as just a movie.
