Interestingly enough, there don't yet seem to be any rules regarding story content. Now, if you're at all familiar with the world of fanfiction, you've probably at least had a few run-ins with content that's sketchy at best, and I don't necessarily mean quality of writing, if you get my drift. So I sincerely hope that the lack of stated guidelines for what constitutes an appropriate story for the fanfiction forum is just because Blizzard hasn't gotten around to typing them up yet, but I suppose that remains to be seen. Still, use discretion when choosing which of your stories to post. I think it's safe to say that graphic sex, violence, and excessive swearing will be frowned upon. Rated T for teen, after all!
Anyway, concerns aside, this new forum will certainly be a wonderful resource for WoW story enthusiasts and writers everywhere. Head on over if you like to read fan stories, and check out what's already been posted!