Well, no, that's not exactly true. Portman herself is fine. No, the worst thing about Thor is Natalie Portman's character, who I want to call Jane Seymour but who was so bland I honestly can't really remember much about her. Jane is a scientist who does no interesting science, and a romantic partner who has no meaningful romantic moments. Her only two accomplishments in the film are transporting Thor and providing him with someone to feel connected to, and when your female lead could be turned from a scientist into a cab driver with a puppy, you know you've wasted a character. Even worse, she doesn't even have an arc; she's a scientist who hasn't accomplished anything at the start of the movie and a scientist who hasn't accomplished anything at the end of the movie, and she's about as interested in Thor at the beginning of the film as she is at the end, which is to say, she basically immediately decides to trust and help the guy who's been hit with a car twice, tasered and sedated, and who is going around claiming he is Thor Odinson, and that he'll give her all the answers she wants as soon as he gets Mjolnir back.
The best thing about Thor also isn't the titular hero, though that's not to say that he's bad. Chris Hemsworth does a pretty good job with a pretty standard hero formula, managing through his acting to do a slightly better job than the script itself. But it's Tom Hiddleston's Loki who steals this film.
Loki is perhaps the most sympathetic of villains, the shining example of what George Lucas tried, and failed so spectacularly, to do with Anakin Skywalker. A mischievous trickster who nonetheless loves his family and his kingdom, Loki is basically met with nothing but scorn and mistrust from the start. He's considered a weasel and a sneak for warning Odin about something, even though it's only Odin's direct intervention that keeps Thor's friends, Loki, and perhaps even Thor himself from getting killed. And no sooner has he assumed the throne than seemingly everyone in Asgard turns against him, in some cases for no particular reason. Watching Loki, one gets the sense that he has no friends on Asgard, that nobody gives this poor fellow the time of day, while people just can't fawn hard enough over Thor who, because someone insulted him, pretty much starts a war.
The choice to make Asgard an alien civilization, rather than a mythical one, was a thoroughly pragmatic one when Jack Kirby and Stan Lee made it decades ago. The realization of that alien civilization in this film is a stunning victory for the effects teams. The city gleams, arranged in an alien but still obvious way, with the odd little jaw-dropping element hiding here and there in the corners. Although I must say, aside from Thor's coronation day ceremony in the beginning, the city does seem rather empty; there are rarely more than, say, six Asgardians on screen at any given time. But perhaps that's what happens to a technologically advanced civilization; maybe they're all inside, watching movies on their 5D tvs and playing the Asgardian equivalent of Halo?
Thor isn't quite as good as the Iron Man films; it's not quite as fun, the hero isn't quite as charismatic, the female lead isn't nearly as interesting. But it's a lot better than Ang Lee's Hulk movie or the two Fantastic Four outings, and it seems like a worthy set-up piece for the ever-nearing The Avengers film. I'd definitely recommend this movie, with the caveat that if you ever need to go to the bathroom or refill your soda, you do so when Natalie Portman is on screen. Trust me, you wont miss anything.