Let's see. We have the killing of a baby girl, an attempted killing of another baby girl, a ghost, and a girl back from the dead. And that's just the supernatural, horror stuff.
The brutal torture of slaves, a sprinkling of bestiality, and several sex scenes, some of them violent. The only reason this novel isn't rated NC-17 has to do with Morrison's language, which doesn't get into a bunch of sensationalist and salacious details. She's pretty blunt about all the "mature" events in the novel, but she still leaves a lot to your imagination.
In case you're wondering, Oprah's movie adaptation of the novel is also rated R. And if Oprah's with us, we must be right. Right?