I found the Oscar Wilde's preface for The Picture of Dorian Gray quite interesting and, as Natalie mentioned, a fun "shove-it" to his critics.
Wilde was widely criticized for writing an "immoral" novel, and in his preface, responds that art cannot be moral or immoral, it can only be well or badly written. The art itself is only art. It cannot prove anything or have any ethical sympathies, or it is no longer art.
In Wilde's day of Victorian literature, however, art was supposed to prove something. It was supposed to teach some ethical or moral lesson. The point of art should be clear and uplifting to the viewer/reader/listener.
Wilde's view was, rather, those who look for meanings and symbol do so "at their own peril." That there should be a diversity of opinion about it.
I find this so fascinating because Wilde strongly believed, from what I can tell, that art should not have a moral point or express some ethical truth. Yet the novel reads much like a cautionary tale, warning readers of the dangers or living a life without thought to the consequences, or of valuing beauty too much.
In the beginning, the painting of Dorian was exactly as Wilde thought all art should be -- beauty for the sake of beauty, with no moral point or lesson involved. Both Basil and Harry thought it his best work. It was pure art.
It's an interesting debate if Basil's obsession began with Dorian himself or the painting. But the art which was so pure, as Wilde thought it should be, is what brought about the downfall of both Dorian and Basil in Wilde's novel.
I find it ironic that the preface and the novel seemed to be almost perfect opposites. Wilde thought art should be admired only for its beauty rather than its moralistic point; yet too much admiration of beauty destroyed his characters, which is in itself a moralistic point. It makes me wonder if the whole novel was satire aimed at the Victorian aristrocacy.