Oh, don't link to this one (except to suggest other people read it--I love getting recs); the ideas here aren't complete.
The goal--and it may not be reachable--is to discuss ratings and the lines between them and terms that could be used to describe them, enough that I can come up with a clever, memorable, iconizable system, where it'll be instantly understandable (at least on a general level), so people will be able to borrow it or adapt & twist it at will. ('Cos if your fandom really believes that "kisses, mouth closed" and "kisses with tongue" need separate ratings... um, go for it.)
Right now, the descriptions are still vague, and the words don't immediately click as "just right," and they don't stick in the memory as a neat coherent system. (I tried to get the initials of the rating-words to spell WISE, but it just wasn't happening.)
Mainly, I want to encourage rating content, not intended readers. And if not suborn the MPAA system, which is probably far too entrenched to go away, at least add some extra options that tell the reader more than "someone thinks this is a teen-appropriate story."