I've been a massive CB fan since I was six, but...
...but this movie doesn't work for me. I know this movie is now considered be-all and end-all of comic book movies, but it doesn't work for me. Before I say why I don't like this movie, I want to clarify that I'm not just a movie snob who doesn't understand the comic book genre. I've been reading comic books my whole life, including The Avengers titles. I started reading Avengers titles after Kurt Busiek's run, and then I came back to the older stuff to catch up with the continuity.
My favorite Avengers writers are: Roy Thomas, John Byrne, Bob Harras, and Kurt Busiek. And despite being a longtime fan of these characters, I did not like this movie (very much). And let's not even talk about the sequel.
My main problems: I don't care for the story, because it's too trivial for my taste. (Punching, shooting, explosions, and nothing more.) I don't care for the characters, because I think they have plastic personalities. I don't like the action, because it doesn't look imaginative and well-constructed. (Again, punching, shooting, explosions, and nothing more.) And Loki wasn't a great as people were saying. He was better than vast majority of MCU villains, but that's not saying much.
These are my main gripes.
I don't hate all MCU films. I do like some of them. The first Iron Man is still a masterpiece in my opinion. Winter Soldier I thought was definitely a step forward from its mediocre predecessor. And Civil War was also good. Heck, I thought Civil War was a better Avengers movie than actual Avengers movies.
Anyway, these are my thoughts.