You don't know, metafandom brought me over here out of curiosity, but I'd like to argue one point in what you've said, and that is the concept of great art.
Who says though what is worthy of great art and what isn't? Critics alone, artists alone, the history makers, or the non-artists that view it?
In it's time, the Venus of Urbino by Titian (I know I spelled that name wrong) for the Duke of Urbino was considered art solely made for the purpose of the Duke's pleasure. Is it art because it's painted, or is it porn because it's purpose is to eroticize the female form?
Throwing the fact that this was a work of fanart aside, would the same picture of two men, one appearing younger than the other, with that amount of nudity and sensuality, still be seen as obscene? Would it mean more being drawn in charcoal or painted in oils? Taking the context of the image portraying Harry Potter out of the picture, what makes this any different than any other homoerotic image?
Admittedly, I'm not even a fan of slash, but because of LJ's actions, it seems to put my writing at risk since I have written stories of a particular pair of 15 year olds in love, and yet my writing style is no different than any romance novel in any national book chain. Last I checked, they didn't card anyone for buying books, but on the internet it seems as if written word is suddenly given a lot more weight.
Also, just because it is fan-made does not mean the quality of writing in most fanfiction automatically on an amateur level. According to the reviews I've received, my stories are just as worthy of being published as any romance book . . . the only problem is that the worlds and characters aren't mine. Myself aside, professional authors have written fanfiction through know pseudonyms. In one quick Google search I found an article
on professional, published authors and fanfiction in which Mercedes Lackey, a best-selling author, admits to writing fanfiction for City of Heroes, an MMORPG.
So really, I have to ask, what is the difference between art and obscene? Is it the medium, the context, or the opinion of one person? How about the difference between professional or not professional? Is it a degree in creative writing, being published by the mass market, or the opinion of who reads the author works?