I liked the idea someone had (Shallot?), about having to put in your date of birth when you sign up, so that you don't get constant warnings, though that would require a lot more vigilance from the people who really don't like slash/adult (catagories?).

Supposedly you could automatically bounce minors from adult stories as long as they're signed in, though that might be a little extreme (but whether or not teens should be allowed to read about sex and violence is a whole other discussion).

Of course, that raises the point: Do you need to sign in before you can read? LJ can keep you logged in. It wouldn't be to much hassle like that.

As for the quality thing... That's a tough one, especially if you're going with a massive archive. Only allow beta-ed fic? (not that, that necessarily helps *shudder*