I think one big purpose the book cover needs to serve is as advertising for the book. It needs to allow that particular book to stand out amongst a large crowd on a shelf, and intrigue someone enough to be interested in opening it up.
Below I've pulled together some of my favorite book covers, which also happen to be fantastic reads.
The Amazing Adventures of Kavalier & Clay by Michael Chabon
Jurassic Park by Michael Crichton
Buddha, Vol. 1 by Osamu Tezuka
What are some of your favorite book cover designs? Do you judge a book by its cover?