Finally saw Avengers the other day. Enjoyed it quite a bit—liked it significantly more than any of the precursor movies except maybe for the first Iron Man.
(No significant spoilers here.)
I had plenty of smallish issues with Avengers, but no really huge ones.
I was disappointed (though not surprised) that it doesn't pass the Bechdel test, but pleased that it had three good female characters, one of whom was quite prominent, all of whom were shown holding their own (physically and/or verbally) when sparring with men.
I laughed a lot (especially at nearly every line said by Tony Stark), and cried a couple of times (especially at one particularly sad scene).
I thought they did a good job of briefly filling in all the necessary backstory for people who hadn't seen Captain America. I think Avengers might be a little confusing for people who haven't seen either of the Iron Man movies (and haven't read the comics), and I suspect that significant pieces would be confusing for people who haven't seen Thor (or read the comics). But I could be wrong. I still haven't seen the latest Hulk movie and have no real desire to, so I don't know whether there's backstory there that was important to this movie, but other people have told me there isn't.
I thought a couple of sequences went on a little too long, and I thought there were a couple too many shots of Black Widow's ass (or, to put it another way, too few shots of the male characters' asses, though it's possible I just failed to notice some), and there were a couple of slightly contrived plot conveniences and a couple of comic-book genre conventions that I don't feel were entirely necessary. But all of those concerns are pretty minor.
Was pleased to see Harry Dean Stanton in a cameo, and of course Stan Lee's usual cameo was fun.
Overall, this is probably my favorite of the Marvel Cinematic Universe films. It's also probably my favorite thing I've seen of Joss Whedon's, with the possible exception of the Buffy musical episode. (I was not a big Buffy fan, never saw Angel, don't love Firefly nearly as much as everyone else, wasn't thrilled with Serenity, didn't like the few Dollhouse episodes I saw, and loved Dr. Horrible until the ending, which I was really put off by.)
I would totally watch a Black Widow solo movie. I've heard conflicting rumors (over the past couple years) about whether that'll happen; here's the latest I've seen, quoted a month ago from an interview with Marvel Studios President of Production Kevin Feige:
We have already planned her next appearances and where to take that character because we believe in it and we believe in her in a big, big way. When will there be a standalone? Both is what we’re heading toward.
I'm not quite sure what that “both” means, but I think it means they're hoping to eventually have her both in her own movie and in movies focused on other people. It sounds like the latter is more likely for now (“we're only going to make [about] two movies a year”), but one of the rumored possibilities is a SHIELD movie, which I would also totally watch.