The A is for "Awesome."
Tony Buschbaum writes about how he wants books for children to have ratings. He recently picked up a book with a reading level of 14 and up, which he felt was, in content, vastly inappropriate for his 14 year old.
On the one hand, I don't particularly see the harm in putting an "L" for "Language" or an "S" for "Lots of Boning" on the cover. On the other hand, I agree with the criticisms in the comments that ratings for movies do in some way act as censorship--cutting material and content for a more favorable rating is common, because the lower the rating, the larger the audience, the huger the profit. That said, that is a fiscal choice to be made, not necessarily an artistic one.
Having no tiny Ombreviations running around, I don't have much of an opinion on this. Thoughts?