Told you there was at least 10,000 words in this topic (as if you couldn't have guessed yourself).
Slate gives a heave-ho:
So, what's obscene pornography? The standards changed with different court cases through the years. The test established by the Roth case asked whether the material as a whole appealed to an average person's prurient interests. A Massachusetts case involving the book John Cleland's Memoirs of a Woman of Pleasure resulted in a three-pronged test: A work was obscene if it appealed to prurient interests, offended community standards, and had no social value. Prosecutors realized, however, that everything could be construed to have some social value. Thus the current standard, called the Miller test, now specifically singles out work that lacks any serious literary, artistic, political, or scientific value.