Jess's Journal has a good post about movie ratings and the absurdity that, Big Night, a movie where characters swear a lot gets an "R" while all the pummellings in Spider Man earn it only a PG-13.
Of course the producers of Big Night HAD to put all that swearing in because adults usually won't go to a movie that isn't at least PG-13, and sometimes not an R, because they tend to assume its a kids movie.
I thought PG-13 was a fair rating for Spider-Man and I think an R would have been too much for a movie with a lot of mostly-bloodless beating up and a death or two. If I'm remembering right, I the Micheal Keaton "Batman" was PG-13 too. I'd say they have equivilent levels of violence and that most teenagers would be just fine seeing either without an adult, though kids might want to sit with mom. Actually, kids might well be bored out of their minds. The CSO is in his twenties and he got antsy during the slow parts of Spider-Man.
I agree with Jess that it is ridiculous that it's easier to earn an "R" through sex than violence. I think that I have mentioned that my cousin took me to "Peggy Sue Got Married" when I was seven or eight and I remember my parents freaking out about it, but I saw it again on cable when I was in my twenties and realized that the sex jokes had gone totally over my head. I had retained the basic plot (Lady goes back in time to high school and wants to do things differently, but then realizes that she was happier doing them the same way. But it might have been a dream. No, wait, it wasn't...)
I do wonder about the language thing, though. If the big concern is that kids will imitate what they see, they are far more likely to pick up the word "fuck" from an excitable Italian than they are to pick up tossing people off of buildings from a bad guy wearing a suit built by his mad scientist father.
Though I hid my eyes as the Snakes on a Plane bit a few choice body parts, the last time I remember being well and truly freaked out when I was in a movie theater was when I watched a preview for "Independence Day" where they very realistically blew up the White House. It wasn't so much that the movie itself traumatized me, but that just when I was getting ready to leave for college, they went and blew up my hometown.
It was freaky, y'all.