I loved it.
First let me say that I have never read the comics, so I only know these characters from the movies. And for me, pretty much everything just worked. So many great moments, and hardly anything to complain about. The only thing I felt was a bit off was how earlier in the movie, Hulk was completely out of control, and later, he was able to sort of function as part of the team. I buy that there could have been an explanation for this ... it just didn't come across in the film. Also how when the alien base was destroyed and all the aliens on Earth died/stopped working--up til then they had seemed to operate independently so that didn't make sense either. And the close-fitting jumpsuits that everyone on the flying aircraft carrier was wearing just made me think of Star Trek. But those are really minor things.
I have to say, I liked Black Widow a lot more after this movie than after just having seen her in Iron Man II. She felt like a much more real and complex character whereas before she basically struck me as a hot gymnast who was good at killing people.
Like I said, haven't read the comics so I know nothing about Thanos, but the reaction of about everyone else in the theater when he turned his head and you could see his face was enough to get me excited about what's coming next.
