Obscenity I always thought referred to personal sensibilities. That is- that it is something, be it an object, action, portrayal or otherwise that offends somebody...but perhaps additionally it is percieved to do so either intentionally, or without intention of avoiding such offense.
There will be certain common areas in most peoples thinking about what obscene is, and according to our current cultural expectations I suppose things like gratuitous or graphic description of various disapproved or taboo acts, such as explicitly sexually related material would be what the thrust of the 'war on obscenity' is about. I don't see any line that I can draw specifically, but I suppose a general principle would be to bestow more value upon books that are not generating controversy for the sake of controversy, or in this case being obscene for the sake of being obscene.
Given that the "right" would be the most vociferous when it comes to this matter, because they have the most to speak about here, I assume the concern is that obscenity a) degrades the 'value' of cultural objects, b) encourages obscene conduct or social irresponsibility especially in children. In a way, there is some merit to the arguments, but I do not think obscenity is in itself supposed to be a factor in deciding where to draw a line. I see no point in banning a film showing real as opposed to simulated sex if it's already R rated (as was the case with Romance). In fact, I see less of a point in banning things if they're obscene than if they're inflammatory and there might be the possibility that it will cause some kind of violent and backward social reaction (and that's hard to judge).
Besides, obscenity or whatever the manifestations might be called are all part of the product of the commercialisation of culture, not in that it wrecks the general output, but we see alot more art literature etc. being common denominator driven. Trying to attack 'obscenity' per se is not addressing the source of the problem, which I think is more the twisted individualism spawned by a degradation of the original liberal ideals.