I don't really have any strict rules... Uhm... If you've read any of the stuff on the site (which I'm assuming you have), then you'll know that the BHFS staff is not very narrow-minded. In fact, we're pretty much just expansive, wild feilds of daisies when it comes to what we'll accept...
I'm not big on CarrotxMarronxCarrot-type things (and that counts shonen-ai, too), but if that's totally your thing, then don't hesitate to submit it. I won't not post your work because it's not yaoi, or because it's a pairing I don't like. I will, however, not post your work if:
* Your grammar and spelling are completely unintelligible. Although I enjoy beta-ing stories, I can't be everyone's nanny. I don't have the time, energy, or sanity to completely revise everyone's story. I usually try to go over stories and check for little stuff, like occasional tense changes and spelling goofs. If your story reads like, "Marrun end Carrit wer totallly difggign ech othur and evrione thot it was rilly sic." I think there's a slight problem with that sentence. So, please use discretion, and spell- and grammar-check your fics.
* If you have a completely tasteless story. There's a Plot? What Plot? fic, and then there's a lemon that's just completely degrading and horrible. (Believe me. I've read some, although, thankfully, not in BH fandom.) Then there are stories that are written just to live out the author's random (and sometimes gross) fantasies. (I've read some of these, too.) Just, come on, people... If you want to write a fic, write something worth reading. For a while I was really pissed off with the fandom world because of all the shit there was. It took me FORever to find some good fanfiction... So, once again, don't write a fic just for the sake of writing a fic... If you're gonna write a fic for the sake of something, write if for the sake of literature... I don't know. And it's not even like I'm all that scrupulous. I just want to see that you've tried, here. Something that you're proud of, something that's good.