John Laird’s review:
If I had to give you a permanent answer right now, I'd be willing to call The Avengers a great flick. I can't say how well it's actually going to hold up over repeat viewings (the first act is slow, and there are a sizeable variety of plot holes that have the potential to agitate), but I do know that it plays out like something that was made by a bunch of people trying really hard to make the right decisions. This results in a movie that constantly feels inspired, like a labor of love, and when I wasn't rooting for the characters on the screen, I was pulling for Joss Whedon and his crew to roll out another scene worth cheering out loud for. If you can't find something/anything to champion in The Avengers, then you should stop going to the theater.
PS - There's no reason to see this in 3D. It doesn't look bad or anything, but you get nothing in exchange for the extra cash you have to pay to get in.