Varsovian:
Hmmmm. It was okay, but not as awesome I wanted it to be. Great moments, great humour, but the overall plot was by-the-numbers. The trailers spoiled all of it, in fact :(
I thought the acting was very good..but I thought the writing really wasn't. Just as you said Varsovian, it was very by-the-numbers...and worst of all, Whedon's use of the by-the-numbers ended up being lazy. There wasn't good motivation for a number of the things done which made a number of sequences come off poorly (the Thor/Iron Man fight, for example).
It was poorly motivated, the stakes weren't believable, they end battle didn't match the villain thematics used up to that point, there were some real clunkers of one liners, there were a couple of really bad music cues, almost no character had anything resembling a real character arc...or even any time to express any complex human emotions (Hulk and Black Widow being two exceptions).
The acting was really good. The fight sequences *looked* good (though often poorly motivated). But the movie wasn't as good as it should have been. It wasn't as good as Thor or Captain America, for example.