I'm going to keep this short and to the point.
Joss Whedon has not created the greatest comic book movie ever made. Avengers: The Movie goes far beyond that. It is so much more than we have seen in any comic book movie before now. Avengers: The Movie captures the essence of everything that makes good comic books enjoyable and conveys it in the form of an amazing movie.
There is action, yes, and it is well-played and well-paced with only a few moments where the CGI is obvious and intrusive. But what really sells this movie is the characters and the relationships between them. Whedon's script is full of these little moments - bits where the characters stop being characters and start being people. Bruce Banner and Tony Stark geeking out in a lab. Black Widow's silent vigil over Hawkeye's sickbed as she worries over the man who saw something good worth saving in her that she never saw in herself. Thor's pleading with Loki to put aside his anger for the good of their family. All these little moments that are nailed by a wonderful ensemble cast.
This is a good movie. It is a good comic book movie but more than that it is a good movie. You don't have to have seen any of the other Marvel Comics movies leading into it - everything will be explained for those who are new to these characters. The characters are new to one another, so there are plenty of natural introductions as the film progresses.
My one bit of advice? Stick around through the all the credits. There are two extras for those with the patience to wait.