The old adage "Don't judge a book by its cover" is widely and utterly ignored by the reading public. People pick up or pass over books all the time based purely on the cover art. Consequently, regional buyers for book chains like Barnes and Noble may double an order of a book that has a cover they think is especially appealing, and cut orders for books with covers they deem unattractive.
What's a writer to do? Never, ever get stuck with a bad cover.
In this instance, "bad" can mean an ugly cover, but it can also means a cover that doesn't speak to the target audience's aesthetic sensibility, or which greatly misleads readers into thinking the book will be something it's not.
Yes, this is shallow and horrifying, but it's how the world works. A bad cover can kill your book dead. So don't let a bad cover happen to your book if you can help it.
Most big publishers have professional design staff, but these pros often work under crushing deadlines and consequently they do make mistakes. Look at their past offerings and try to get a cover approval clause written into your contract if you have any doubt that they'll give you a good cover.
Small-press publishers may or may not be run by people with good art sense, but they'll generally be perfectly willing to work with you if you approach them politely with suggestions. If you're working with a small press and they're not receptive to your suggestions, something's wrong.
But what if you're not sure if you know what separates a good cover from a bad one? Then take some time to learn a little about the basics of graphic design and typography. Being "artistic" is as much a learned skill as it is a natural instinct; even if you think you're art blind, you probably can learn the basics of good design.
And if after Art 101 you're still convinced that digitally-generated covers featuring bluish Poser figures trapped in the Uncanny Valley look just fine ... make friends with an artist who likes to read the kind of books you like to write. They can help warn you when a bad cover is about to happen to you.
wertperch says I've watched people in airport shops turning away from something because they "prefer this cover", or "this looks better!" Stupid, stupid people.