I am assuming you either don't care about spoilers or you already saw the movie. Last chance for fluffy bunnies.
First I loved this movie. Marrying six characters, four of them who already have had their back stories written out and keeping them all interesting is no easy feat. The first act of the story was more drawn out then some of the previous films out of a need to establish all six main characters, Loki and Shield for the audience. One regret I had was that Agent Maria Hill felt a little interchangeable as a character, it felt as if any random soldier could have filled that part.
On the reverse side of that the Black Widow subverting the helpless damsel trope was brilliant. I find the idea that in rage someone exposes themself as much or in these cases more then the "victim" is an interesting and empowering idea.
This brings me to one of the best things Joss managed. Every single character got a moment where the audience literally applauded. Hulk stole the show though. "Puny god." There was less of an arc with the main characters. You can't move them forward too much because of the limits of the franchise. Ironman 3 is in some form of production now and Thor and Captain America sequels are in the pipeline. Still Steve Rodgers ended the film more comfortable with the world outside. Tony Stark and Rodgers push each other in the development so that Stark ends up more selfless and Rodgers more willing to look at the world with out the idealized view of his past. Thor ended up with very little development. His interactions with Loki were limited, in my opinion because if he was the proactive force the movie could have ended up being Thor 2 instead of an ensemble. Bruce Banner continues the journey echoing Rodger's acceptance journey and Stark's quest to do the right thing even if that sacrifices himself or in Banner's case his need to keep control.
Whedon's trademark is his witty banter and it keeps the very expected journeys from becoming trite. The villain's journey is one of the more underdeveloped. Loki's backstory and motivations are implied with his interactions with Thor but not explained. Whedon's other trademark is to let you grow attached to a character so that when he's killed you feel it. Agent Coulson's role was just that, a brutal moment just after a moment of strength. His last words were a very meta reference to the heroes needing his death to motivate them. Fury twist the knife even further taking the death to the point of mellow drama when he
doctors the cards with blood and flings them all on the table.
The third act of this was dynamite. Let's be honest. We knew generally how this was going to end. You can't have a movie called the Avengers with out knowing that our heroes are going to team up and defeat the bad guy at the end of the day. The brilliance of the third act is that even though you know they will win you don't care. You don't need the mystery to keep engaged with the story. We know Hulk is going to come back. We know that Tony isn't going to die, although the earlier death allows you to worry about Black Widow and Hawkeye who don't currently have movie franchises behind them. But the action is good, the lines are quotable and hilarious, and the ending satisfies the story.