I give it an A. It's not quite perfect, I would have liked a bit more action, but what's there there's nothing wrong with. Great characters, acting, set pieces, music and design.
And a great plus for me as someone who didn't really know about the comic of Thor before I saw this movie, but love Norse mythology, I really liked how they injected the mythology into the Marvel universe. Good stuff.
I'll be recommending this one to people I know.