What Makes a Great Ebook Cover?

What makes a great Ebook cover?  I'm in 'cover' mode for my new Ebook coming out shortly and I was wondering if any and all had some thoughts on the subject of Ebook Covers.  It's our first impression before they've read word one.  What makes them great?  What makes you want to read whats underneath?  Leave your thoughts and your covers and let's discuss.