Joss Whedon is a master story teller. He seamlessly stitches four power threads together in the form of Thor, Iron Man, Captain America and the Hulk, capturing the spirit of their independent movies and retaining it in this ensemble film while greatly expanding the characters of Nick Fury, Hawkeye and the Black Widow. Keeping all of these things juggled while also handling a powerful and scene stealing villain like Loki, giving an action packed fast paced storyline which allows all the heroes to shine and have a reason to be there is utterly fantastic.
This is one of those movies that I can enjoy watching again and again. It also handles the transformation from a world with superheroes before to a world with superheroes after, including the reaction of the political establishment. The avengers protected and then literally shook the world and the great powers sat up and took notice. Joss Whedon maintains some of his themes, but broadly speaking makes the whole thing work.
It is fun. It is well written, visually stunning and well worth your valuable time.