The characters are treated with respect, as filmmakers have realised they deserve, but Whedon still manages to have fun with them without it becoming overburdened by clever Whedonesque dialogue - I heard it said that it's a superhero movie with Whedon touches, not a Whedon move with superheroes in it, and this is definitely true.
I'm trying to think of any way it could be better and anything I've come up with is nitpicking. Hawkeye is a little underused, but compared to the others he's probably the least interesting so this is understandable to a degree.
Hulk gets a great showing, and whilst the final battle does contain lots of CG - to be expected - it feels like less of a CG monster battle, as both of Hulk's recent outings have somewhat descended into.
The appearance of Thanos in the mid-credit sequence at the end suggests the next film(s) could take place largely in space, an interesting place to set it when the filmmakers seem to be trying so hard to keep these films as down to earth (literally as well as figuratively) and realistic as possible, but the team behind this one could definitely pull it off.
Now, I have to go and buy the Blu-rays of all the flicks leading up to this one that I don't already own... Damn you Marvel Studios...