Ratings by themselves don't seem to be too much of a problem, and it would probably make the process of blacklisting certain sites for schools or parental control programs easier. Mandating that such filtering be offered by ISPs might not be a horrible idea, so long as services that use or disuse the filtering have no cost difference to the consumer, and that both types are available.
I also think that implementing the dot triple-X Top Level Domain is at least as good an idea, since the issues it raises are already coming up in this "ratings system" debate anyway.
So far these proposals sound pretty measured, unlike Australia's. Let's all just keep in mind the reasons why "the people who created the internet ... talked very deliberately about creating a space that Governments couldn’t reach," as Burnham put it.
As to the removal of material, I can't speak for social networking sites like Facebook but places like Photobucket, Imageshack, and Youtube can be pretty prompt in removing sexually explicitly material from the site of their own volition. I'm not sure what benefit there is to a legally-imposed deadline for removal, in broad terms. What does this do for sites that already try to enforce their ToS in that regard?