The only things more important than a cover when it comes to whether someone will pick up a book are trusted recommendations and whether the person enjoyed the previous book/current author. So a cover is pretty darn important.
Essential elements include an image on the front that not only grabs attention but sells the contents of the book. The attention grabbing is important so it has a chance to sell. And I say sell, because describing or informing the potential reader what's in the book isn't enough--there's too much competition. (Of course, if you have a big name, you can fill half the cover with it and still sell books.)
Once that is taken care of, a compelling block of text telling the reader why his life will be incomplete if he doesn't read this book helps those on the edge. And a few endorsements/testimonies never hurt to add a bandwagon argument to the mix. People are suckers for bandwagon arguments.
The image above is a combination of digital and traditional illustration techniques (can you identify where I used which?). The color scheme is one I've played with in the past and wanted to try again, but in a more developed way. The first time I felt it was too harsh. I think I've figured it out so it's striking without being overpowering. But that's just my opinion.
Let me know what you think!