I know the idea of genres has always been somewhat controversial because of the difficulty in pinning a book into a pigeonhole of a classification, but I do think genres are necessary to at least give broad approximations of what a given book is about. Without them, it would be difficult to quickly describe a certain novel or even to organize books in something like a library or in an online catalogue.
Despite this obvious usefulness, I still feel that the existence of genres can sometimes be a bad thing, and this arises primarily from stereotyping. This happens at one point or another to most people, and for me it happened with science fiction. I found that I just, in general, do not like "traditional" science fiction, and so for the most part I have avoided such books, sticking mainly to sci-fi like Malinda Lo's "Adaptation" instead of, say, something by Asimov.
Recently, however, I was persuaded to read Ender's Game, which had many of the elements of "traditional" science fiction - spaceships, aliens, a nearly all-male cast, a future society, and so forth. Yet despite this, I found myself enjoying the book a great deal. Because I had stereotyped the genre of science fiction to be something dull with little character development, I prevented myself from trying other works that I also might potentially enjoy.
So, what do you think about the existence of genres? Are there any genres you try to avoid?