To be honest, I'm not sure I'm the best person to give a critique of Avengers since I'm a massive fanboy of both Marvel and Joss Whedon, but since the wolf boy did ask for it...
See it, it's brilliant.
It pays off pretty much everything they've been laying the groundwork for in the earlier films. But, as my girlfriend (who previously Marvel credentials top out at falling asleep during Iron Man 2 and managing about the first 10 minutes of Thor) can attest to, also manages to remain completely accessible for anyone coming in to Avengers fresh. It also manages that balance that Joss Whedon does oh so well of action and comedy, with neither seeming forced.
I also had to smile at all the people leaving the cinema as soon as the credits rolled. Everyone knows Marvel films ain't over till the big tease :)