Disclaimer: This review is coming from a biased DC fan. My views may not be completely accurate to the general audience. The nitpicking is more than occasional. You've been warned.
To be honest, I hadn't been completely interested in the Avengers movie, since Ed Norton was dropped. But that was more of a personal preference, than a view of the film. Still, I went into the theatre with a semi-open mind, only to be disappointed.
Tone My main problem with Marvel is that they try too hard to make their characters relatable. I'd never want to read about an alcoholic Batman, a wife-beating Superman, or a Wonder Woman with an eating disorder. But again, thats personal preference.
Going into the Avengers, it seemed like the implied grittiness took away from the other aspects. If they wanted to make a family film, they should've made a family film. But, instead,
they tried to mix both together into a long, drawn out mess.
Plot The first half of the movie was forcibly drawn out. As much as they could have done with it, they stuck with two narratives (Hulk & Ironman). They basically discarded everything that built up to the Avengers, and focused on Stark's snarkiness. Instead of showing how Captain America had been holding up since First Avenger, they showed him with a punching bag. Instead of showing what Thor had been up to, they had him glide down to earth with the Mjolnir, 20 minutes into the movie.
The second half of the movie was just a filler. Most character deaths were completely forced and unnecessary (don't even get me started on Coulson). It seemed like all the buildup in the movie, was just so that Hulk could smash things. Everything else was just Snarky Starky.
Characters The main problem with team-up movies is that not every character will get enough screen time. To put it bluntly- too much Stark, not enough space. He didn't need to be the focus of the movie. Isn't Captain America supposed to lead the team? Only saw that portrayed once. Thor might be good as a supporting character, but that doesn't mean he should be cut out completely. And to put it bluntly, Hulk was...smashing.
I honestly think that this movie gave no more hope to superhero teamup movies, than did the X-Men films. In my opinion, The Avengers was average. But what do I know? I'm just the 500 pound, DC biased gorilla in the room.