The story in Avengers is basic - Loki wants to rule over Earth and a disparate bunch of superheroes (Iron Man, Captain America, Thor, Black Widow, The Hulk and Hawkeye) must band together to fight him and his army.
Joss Whedon works his magic to maintain the balance amongst all the characters (with nods to their respective movies) while blending action, humor and drama in a way that didn't feel false or forced.
The climactic action sequence is one of the most breath taking rollercoaster rides I've seen in a number of years.
I have no complaints over the acting though I must give special praise to Tom Hiddleston - he was electric as Loki - arrogant, petulant and vulnerable at times as well.
Everyone in the cinema had a great time - Joss Whedon should be pleased with the spontaneous cheer that erupted at a vital moment.