Steven Zeitchik examines the troubled life of the NC-17 rating, conceived in the late '80s to replace the X rating and "usher in an era of mainstream acceptance for films with serious adult themes." That didn't work out so well:

Now, even as basic cable is constantly pushing into ever-more steamy and violent territory and a wide variety of pornography is easily available on the Web, movie theaters are practically devoid of formally adults-only films. The number of movies released with the NC-17 rating has plummeted; those that do go out with that stamp do little business at the box office. The reasons are clear: Some theater chains, including Cinemark, the nation's third-largest circuit, won't play them. A number of media outlets, particularly newspapers and television stations in more conservative states, won't accept advertising for them. Wal-Mart and other retailers won't sell copies on DVD. Now at 22 years old — the same age as the X was when it was retired — the NC-17 is seen inside Hollywood and beyond as ineffective and broken. But no one can agree on how to fix it.

Another problem is the fickle nature of film ratings:

Though many moviegoers know, for instance, that multiple uses of the F-word can turn an otherwise PG-13 movie into an R film, the boundary between R and NC-17 is much less distinct. The MPAA says NC-17 ratings can be based on "violence, sex, aberrational behavior, drug abuse or any other element that most parents would consider too strong and therefore off-limits for viewing by their children." The group says an NC-17 "does not mean 'obscene' or 'pornographic' in the common or legal meaning of those words, and should not be construed as a negative judgment in any sense. The rating simply signals that the content is appropriate only for an adult audience." But many adults won't go to an NC-17 movie, convinced that they're going to watch smut.

(Hat tip: Ebert)