The Picture of Dorian Gray was a highly controversial book when it was released, which seems strange now. The clear message of the book is that you can't live an immoral life and not reap what you have sown. That seems as moral a moral as any book could ever have, but people in Oscar Wilde's day objected to the graphic depictions in the book as well as the apparent homosexual undertones.
The best part about The Picture of Dorian Gray is that it's still something to talk about more than 100 years after it was written. While the horror may be dated, the tale is not, and its moral implications could be debated nearly endlessly. I also loved the fact that Oscar Wilde couldn't help but include bits of his trademark humor in a horror novel. All in all it's a very good book that everyone should read at least once.