I thought the Avengers was really fun... I wasn't blown away by it. It was good. About the same as the Iron Man 2, Incredible Hulk, Captain America and Thor movies. Fun, but not spectacular movies. I wouldn't say it's anywhere near what The Dark Knight was if we're measuring super hero films. I felt The Avengers had some fun moments, but ultimately it was a little forgettable. That's just me, though. Classic popcorn flick blockbuster.
For the summer films I'm still most excited about the third Nolan Batman movie. I don't expect it to "top" Dark Knight. I don't know if it has to, though. In many ways, I liked "Batman Begins" more than "Dark Knight". The JOker obviously put Dark Knight over the top, but Batman Begins deserves more credit. As long as this third film wraps up the story nicely (it's a given Ra's al Ghul returns, right? He has to. Makes too much logical sense for the 3 part story arc) I think it will be fantastic.
EDIT: Just checked the score for the Avengers on metacritic. 69. Yup. That seems just about right. Good fun movie... not a great movie. I wouldn't say it's "disappointing", though. I got what I expected out of it. It was fun to see all the heroes together. Side note: When I saw "Thor" in the theater the movie ended with a teaser trailer for "The Avengers". Everyone in the theater lost their minds when that teaser trailer was shown. I didn't get it. I thought Thor was "blah". I wasn't sure why anyone would expect "The Avengers" to be better. If Thor kinda stunk... I wasn't sure why anyone thought bringing all the characters together was going to make the movie vastly superior. Cap America was blah, Iron Man 2 was blah. Incredible Hulk (with Ed Norton) was blah. Thor was blah. Stick them all together and you have a nice little popcorn flick. It's great fun to see Bruce Banner and Tony Stark interact, but as a movie that's all it is... fun. Cool action scenes. Some fun interactions. *shrug*. I think I was more engrossed in "X-Men First Class" (by no means a perfect movie either)... is that weird?