Lots of bloggers have been talking about covers in the last month-plus (cf Lou Anders and the Penguin blog, among others. And Irene Gallo's blog is a must-read for anyone interested in sf/f cover art and artists, and illustrators in general.)
Personally, I'd love for everyone to understand that the cover is to sell the book. To the reader, yes, but first and foremost to the buyer at the store and/or chain level--if they don't like the cover, it stands a much smaller chance of being seen by anyone else. So the cover is designed primarily to appeal to the buyer. After that, we want it to grab the readers' attention. And their hands. And particularly their wallets. That being said, we do want a gorgeous cover that will also reflect the book. Sometimes they succeed; sometimes, unfortunately, they don't.
For example, when Eos first started, many people were saying they were embarassed by the girls-in-bikinis-riding-spaceships-and-dragons look of many sf/f books. So we did gorgeous, cutting-edge graphic illustration covers, with some of the top illustrators working at that point. And many of the books, well, tanked. Painfully. So we went back to the more traditional style of cover art.
On the other hand, when Harper switched cover looks on Terry Pratchett's books, moving away from the Kidby cover art that the UK uses to more graphic designs, there were loud howls of outcry and protest from US fans. (This is still a popular question/coment at conventions.) But the sales went up with every book thereafter (including the backlist repackages), until last year when THUD! hit the New York Times list last year for the first time in the US.
So there isn't always a clear answer, and like Lou and Irene, I am carefully skirting the issues of what can and cannot be said. But overall, the goal is to get a cover that's gorgeous, eyecatching, and will sell the hell out of the book. Sometimes we get two of the three, and sometimes we hit the trifecta. But the aim is always to do all three.
Speaking of, here are some recent and upcoming Eos/Harper sf & f covers that I am particularly proud of and excited about. They cover a wide range of title and styles, but they all make you stop and take a second look.