Am I in a very minor minority here?
First, let me make the dreaded confession - yes, I HAVE pre-judged a book by a cover - we ALL do it - it's that visual thang that's hardwired into the human brain. The cover is the first thing we see on a book, and come on, be honest, if a cover leaves you absolutely cold would YOU hold out any hopes that the book underneath it would enthrall you? Didn't think so. Dreaded confession two - I've been surprised and astonished any number of times when, being persuaded to read a book whose cover did not appeal, I actually found the book itself not only acceptable but often absorbing.
Yes, I can be shallow that way.
From under the author's hat, I await the book covers on my own books with anxious trepidation because I know first-hand of reactions much like the one I have described above. Some people who might have loved the book will never pick it up because the cover doesn't trigger their "pick me up" impulse. A recent reviewer of "Spellspam" bemoaned the cover because to her it would have meant that she would never have picked up that book if she had had the choosing of it - and she thought the cover was bad. There are people close to me who agree with her. There are other people who adore that cover. Personally, I think that the three books makes a nice set, cover-wise. But there you have it - there's at least one person out there who WOULD NOT HAVE PICKED IT UP if she had tripped over it in a bookstore.
Another case in point - "Secrets of Jin Shei" was written and conceptualised in my own head as high-concept alternate history or historical fantasy book, along the lines of Guy Gavriel Kay's "Tigana" - but it was marketed as mainstream, and given a mainstream-y cover. What did that mean - well - it meant that I was sliced by the double-edged sword of having potentially higher exposure (the book was shelved in the mainstream section, and while fantasy readers WILL be seen in the mainstream section mainstream readers would not be seen DEAD crossing the other way) but by the same token those people who would have picked it up and loved it as what it was originally intended to be may have never heard of it or seen it and might never have picked it up judging it by its mainstream-y cover, and those people who did pick it up based on its bookstore placement and its mainstream-y cover might have been deeply disappointed and even annoyed by the fantasy elements in what they believed to be a mainstream historical novel.
So. Slapping it on the counter and staring at it. Book covers. How do you judge books? Do book covers make a difference? To simplify the thing completely, the same book was published with a pink cover and a blue cover would it matter to you which?
Do unmet expectations, as based on what you were led to expect from a cover, ever sour your opinion on the author - who had nothing whatsoever to do with what sort of cover got slapped on his or her book...?