Alas, this is what consumers (including you and I) do every day.
Book covers do a number of things for us, but in my opinion, the three most important are (in no particular order):
1. Tell us what the book is called and who wrote it;
2. Give us an idea of the book's genre (thereby already telling us whether we're likely to enjoy it);
3. Provide an iconic, interesting image, so as to—as Seth Godin puts it—"tee up the reader so the book has maximum impact" (see below).
Human beings are visual creatures. This means that any factor that interferes with the above three points can literally tank a book's sales.
A couple of examples.
The above image violates the first "rule" I outlined, namely that the book's cover should communicate the book's title and author. In this case, does that matter? I don't think so—it's clear (at least, to me) that the book is fiction, probably literary fiction, and so I'm likely to pick it up. (This may be an artifact of my having worked in the industry, though, so I won't assume everyone makes my intuitive leap.)
The more important point is that the cover is striking, iconic, simple, and somewhat mysterious. This is the key to driving sales, as noted by the inimitable Book Ninja (who is, in turn, quoting this original post by Seth Godin). Both Seth and the Book Ninja correctly point out that the book's cover is designed to drive readers to the story—front cover to back cover to flap/jacket copy to book—and a stumble in this first step (i.e. a bad cover) can translate to very poor sales.
What about this cover?
In my opinion, this one doesn't follow the second "rule"—communicating genre to the consumer—and may have cost the publisher sales as a result. If you didn't see "DEAN KOONTZ" emblazoned in forty-eight point font across the cover, would you identify this book as horror/thriller? I doubt it. I, for one, would immediately mentally classify it as contemporary romance and never give it a second look, since that's not a genre that interests me. I mean, even if you do read Dean's name on the cover, you may assume he's jumped ship and written some kind of romance novel—quite an assumption, I think, but I couldn't blame you for making it.
The cover is fairly striking, but you can tell that's not enough; it's also got to appeal to its target audience. I'm sure you've all seen the cover trope commonly called "the clinch," the hallmark of a romance novel, and this post over at Jezebel explains the intimate relationship between cover and genre. Consumers interested in romance buy romance and pick up anything that looks like romance; the same goes for consumers of children's books, literary fiction, business books, and so on. You can't swap Harry Potter's cover art with that of Freakonomics—even if you properly label each with the correct author and title—and expect sales not to be impacted.
While we shouldn't judge the content of a book by its cover, the fact is, we can and we do, and publishers are betting on the fact that we do. So, once you've sold your book to said publisher, how can you make sure your book isn't tanked by an inappropriate or boring cover?
Well, to a certain extent, you can't. While there are certainly authors (mostly hot shots and celebrities) who have significant say in terms of their cover art, the vast majority of book covers are engineered in the publisher's art department with little or no input from the author. (You may begin to suspect as I explain the industry more and more that, once you've sent your MS off to the agent, fewer and fewer things are in your control. If so, you are correct.) It's also worth pointing out that Koontz is one of said hot shots and still is not immune to the occasional bad cover.
However, he has dozens of books to his name, and this could be your first one.
The good news is, I've very rarely heard of an author who absolutely loathed his or her cover and whose book went to print regardless. While no one from Big Name Publisher is going to come to your house with color palettes to have you pick out the color scheme for your next book's cover, they will run the cover by editorial, sales, marketing, publicity, your agent, and you, and they will most likely be willing to hear you out if you have a major grievance.
In a nutshell, then:
• People pick up, judge, and occasionally buy books based on their covers.
• Publishers assume this.
• Good covers communicate data to the consumer—author, title, genre—as well as excitement/intrigue via the use of iconic, generic (generic meaning expected/familiar, not boring), and interesting images. This is one of the major factors that translates to good sales.
• You want good sales. Therefore, you want a good cover.
If your publisher is showing you cover proofs for the first time, look for these signs. If you would pick it up off the shelf based on the cover alone, that bodes well for you and your book.
I leave you, then, with these: the best book covers of 2008, as judged by the New York Times Book Design Review.