Director Joss Whedon pulls it off, mostly. The plot is actually quite simple. Loki makes a deal with an alien race to lead them in an invasion of earth. Nick Fury has to get earth's mightiest heroes to team up and stop it. That's the whole plot.
The first hour or so of the movie moves at a slightly slow pace. Its about getting the heroes together, and then getting them to stop quarreling with each other and get them focused on the greater threat. A big portion is set on the SHIELD aircraft, giving the scenes an almost "kitchen sink drama" feel, which should have been avoided. This first part of the movie, with tighter editing and a quicker pace, could have been shortened by about 15 minutes, and would have made the movie better overall.
The second half of the movie is what we came to see. The Avengers taking on the invading aliens and kicking ass. The special effects are excellent, as are the action sequences. Now a break down to the main characters.
Captain America: Chris Evans does a great job in the role, and only hints at the fish out of water aspect (needless to say, that will play a much bigger role in Captain America II, so Whedon couldn't do too much with that here). As expected, Steve Rogers ultimately becomes the de facto leader, figuring out the Avengers' strategy.
Iron Man: Robert Downey Jr once again plays the role with a comedic flair, giving the character far more personality than he ever had in the comics. You can tell he's probably having the most fun in the movie. If he hasn't already, Downey needs to get on his knees to Michael Keaton and thank him, because there is no way Downey ever would have even been given a second thought for the role of Iron Man if Keaton had not blazed the trail by playing The Batman.
Thor: Chris Hemsworth also picks the role up right where he left off from his solo movie. Overall, Thor plays very well in this movie. He gets some spectacular special effects calling down lightning, and his flying scenes are very real looking. It may be because unlike Superman and Iron Man, all we get are glimpses of Thor flying, which makes the viewer want to see more. My one complaint is he never wore his helmet. Loki got to wear his big antler helmet for a lot of the movie, but where's Thor's helmet? Another complaint is it seems in post production, care was not taken to make the plastic parts of Thor's, and Captain America's costumes (including Thor's hammer) look like metal as they did in the solo films. Here, it does look like cheap plastic.
Hulk: it is in a movie like this where the character of the Hulk actually becomes interesting. In the two prior solo Hulk movies, with the big green goliath at the center of the plot, it gets kind of repetitive. Here, where the Hulk is a supporting player, the character can shine much better than in a solo film. As for Mark Ruffalo taking over the Bruce Banner role, I think he falls in the middle. Not as bad as Eric Bana, but not quite as good as Edward Norton.
Hawkeye and Black Widow: In the original Avengers line up, it was Ant-Man and Wasp. Good call in using Hawkeye and Black Widow instead. Hawkeye spends most of the movie under Loki's spell, and Scarlett Johansson does very well with her character.
Nick Fury: Samuel L Jackson plays the role that was tailored for him by the Marvel Ultimate Universe creators. But really, come on... we all know the only real Nick Fury is David Hasselhoff.
OK... I'm kidding on that one.
Loki: Tom Hiddleson really excels in the role, evolving the character from his appearance in Thor as being far more evil and cunning. Plus the Hulk's treatment of Loki got the movie's biggest laughs and cheers.
Overall, The Avengers is an excellent film, despite being a little slow in the first half, and Disney-Marvel hit a home run they can be proud of.