Don't worry, zero spoilers. I'll just say that it was worth the wait. The movie delivers on the promise we were given during Iron Man in 2008.
How do I wish Warner were serious like this about their DC movies. I know most people love Nolan's vision for Batman, but I'm in the minority that thinks Batman could be done different, without having to sink into the awful camp of the Schumacher movies or the all-style-zero-substance of Burton. Other than Nolan's Batman we've been treated to a Superman movie that was the "chick-flick" of superheroes, a semi-sequel to a movie from the 70's... oh, and a Green Lantern movie that was a mess, I rented it, and couldn't sit through it. I shudder to think about what Nolan, Snyder and Goyer might have come up with to "ground" Superman.
As a general rule, I dislike superhero movies, they almost never hit the mark. But, starting with the first Iron Man, I felt that the Marvel Studios movies (with the exception of Incredible Hulk) were aiming to make movies that were not ashamed of the source material, that actually embraced it and had fun with it.
My hope is that The Avengers marks a definitive shift in the approach this type of movies are made, and that the school of thought that began with X-Men (how would superheroes work in the real world) fades out.
Admittedly, a JLA movie, if done right (not like that cancelled project where the actors cast were all 21 or 22 years-old), would dwarf what I just saw in the Avengers... which might be cost prohibitive. The power scale of the DC pantheon is on an entirely different level. The other thing is that a JLA movie will now be seen as Warner trying to play catch-up with Marvel.
/rant over.