Some will say that there are technical considerations—the quality of rendering, the beauty of the language, or the composition of the scene make a difference between obscene and not, porn and art.
Personally, whether it's prize-winning literature, a cheesy film, or a fashion spread, my impulse to name the obscene, to pick up a black marker or start scribbling protests in the margins, depends a lot on who wrote, or painted, or filmed it. After all, you don't want your vision of the world hijacked by just anyone, even for a moment or an hour or a few hundred pages. Do you?