Let's discuss things in a reasonably spoiler-free fashion. You have to consider that the only film in this canon that I've seen was Iron Man, which I liked quite a bit. Then little girls arrived, so I haven't seen any of the other films (although DVDs of Captain America and Thor lurk downstairs somewhere). Thus, to the extent that we're relying on plot elements introduced elsewhere, I'm adrift.
That was mostly ok. Apparently, the main McGuffin comes from one of the other films, but I can get by that. And I know enough of the history of the comic-book Marvel Universe that I can pretty easily follow what's going on and spot the changes and the resonances.
I miss Hank and Jan from the original team, but I can understand why you'd end up replacing them with Hawkeye and the Black Widow, especially given that Bruce Banner seems to be filling in as the resident biochemist here. And I really enjoyed most of the character bits. I especially enjoyed it when the Hulk met Loki. (This may reflect how things are going at work. It's hard to say. :) )
But I have a problem with the fact that Loki seems to be an idiot while Nick Fury seems to be playing a much deeper game than I would ever give him credit for.
It was great fun to watch. It just feels a little empty somehow.
And that's a shame.