I'm sure I have passed by many a great book for the sake of a bad cover. - Patrick Flanery, NY Times Book Reviews 
There is more to a book cover than stock photographs and some typesetting. The indie publishing sites are covered with volumes that have been thrown into the howling hurricane that is the ever shifting book world with poor cover art. In this market, as in the bookstore, you have one chance to grab the paying reader with a great cover. Remember, you have only one opportunity to make a first impression...
Your cover is your first chance to tell a pictorial story of what is inside the body of your book. Not a literal interpretation but a one shot grabber that will make a buyer pick up the book and read the synopsis.
UNAUTHORIZED ACCESS by Best Selling author Andrew McAllister. Judging by the cover, what is the story? Let's take a look and see. There is a city, a frightened girl and a bold title that indicates that there is a reason for her to be so frightened. Great color, simple agitation and a visual that invites the reader to embrace the drama they know is unfolding in the pages of this wonderful thriller.
A note of interest. This was not the book's first cover. Upon first release the book was well received but the original cover was, well, scary. Women readers were reluctant to buy it because they thought it would be too violent. After evaluating the book's theme, the new cover helped it become extremely popular among female readers. From good to great, it's all in the packaging.
95% of all readers admit the cover is the reason they first look at a book. 
IMAGERY: The photograph or illustration must be more than a generic image. The selection of a free standing image or the creation of a composite picture should portray the emotion or the drama in the story. A balance of tease and story entices the potential reader into wanting more.
FONT:  Text on the cover should be balanced and appropriate to the image. The font style and positioning should help create the drama promised by the image. The use of contrast is essential to create dynamic tension and invite the reader to pick up or look further into the book.
This is an actual cover I found on Amazon. The names have been changed to protect the innocent. This graphic misses all the marks to be considered "good."