I like The Avengers. It was good, but I don't think it was great. It was not a movie I am going to watch again.
The first problem is that it is Joss Whedon. I love almost everything he does. So the director of The Avengers had some big shoes to full -- Joss's prior works.
The problem with movies with Superheroes is that you know what is going to happen. The Superheroes will save the world. The suspense is gone. It addition, they are thinking huge money, and as someone pointed out, Joss wanted to kill someone, but the studio wouldn't let him. It wasn't the Joss I know and love. Without listing any spoilers, the main characters that die in Buffy and Angel and Serenity are brilliant to the plot, and very real. To me this was just a typical big budget Superhero movie.
IMHO, this does not rank among Joss's finest work. I would rank "The Body" and "Not Fade Away" as much better. Those I do rewatch over and over.
This movie does not hit "great" for me. It was good, and it was enjoyable 2 hours, but I just didn't feel Joss in the movie. Weird, eh? But if you told me it was Joss after the movie, I would have been surprised.
BTW, yesterday I just rewatched "The Gift." For me, that is great Joss Whedon. The Avengers is not. 🙁