Book Covers

The question that always crops up on discussion threads, is how much importance does the cover of a book play in whether or not a reader purchases it?  What constitutes a good book cover, in your opinion?

 I've been pleased with most of my covers and recently added them to the "rate my cover site.  I've been amazed to find that the covers I thought were the most attractive and depicted the story to a tee received the lower ratings.  Evidently, I have no idea what makes a cover appealing. :)

I've re-released a few of my older works and I'm amazed at the different depictions between two cover artists who received the same input from me.  The one on the left is my new cover, and the one on the right is the debut cover.

The same for Prairie Peace. Isn't it amazing how the idea can evolve into such different covers? This story is about a newlywed wife who finds herself stranded on the prairie, miles from civilization and depending upon an injured Indian to help her survive when her husband fails to return from gathering supplies for the approaching winter.  The new cover is the top one and the bottom, the old.

Then, look at the transformation between the two covers for Sisters in Time, my time travel with features two women from different eras who switch time and bodies for a trip they'll never forget. 

 And last but certainly not least, my coming revised copy of Beside Myself.  Here's the new cover that will be appearing:

 

So, what makes a good cover in your opinion?  I'd love to know.