When I kept hearing that the movie "Thor" doesn't take itself too seriously, I thought, "okay, that's a warning that there's going to be a lot of goofy, cornball humor." Thankfully, it was kept to a minimum.
Personally, I don't understand why a movie should not take itself "too seriously"; Will some movie-goers suffer spontaneous human combustion if a movie doesn't have one-liners in it, or doesn't have a fart joke, or doesn't have a cute little cg character that says idiotic things during tense moments? But I digress...
I give Thor an "A-". First of all, to bring a character like Thor to the big screen, without it appearing silly, is an immense accomplishment. Also, as others have noted, the acting is very good.
For me, there was only one major flaw in the movie: The Thor/Destroyer fight. Like the final fight scene in Iron Man 2, it ended way too quickly. I don't mind Thor kicking butt, but it's rather anti-climactic when he does it so easily. I mean, c'mon, comics are about action...so give us a longer fight scene, with each character getting in a few licks.
After the fight scene with the Destroyer, Thor walks out of the fight-dust-cloud and Natalie Portman says "Is this how you normally dress?" This is a prime example of how a one-liner can detract from a movie. In an unneeded attempt to "not take itself too seriously"--and get a one-liner in there, the battle scene is trimmed down.
So, if the Thor/Destroyer fight was extended, and the "is this how you normally dress" one-liner removed, the movie would have been a solid "A".
But the movie was very, very good. Definitely better than the uninspired, mediocre Iron Man 2, but not quite to the level of Iron Man. And it's a great precursor to Avengers, without sacrificing the story that you're actually watching in the theater (i.e., not weighed down with Avenger teasers).
Thor finally gets his day in the theaters!