This is a great topic but I think you need to distinguish between a GREAT cover design for PRINTED books and one for E-BOOKS.
The visual requirements are NOT the same: for stuff on paper, the sky's the limit. You can let your imagination go in terms of colors (flying, of course), subject (cropped bodies, milling crowds, endless rooftops) and even texture (bumpy, raised, whatever seems right, a plate of overcooked noodles being the exception).
For an e-book, you have to remember the cover design is going to be reduced to POSTAGE STAMP size on your Kindle or whatever e-reader your readers might be using (the iPad being presumably - logically - the only exception, but then stats are out: most people don't use their iPad to read and have kept their Kindle for the purpose, even after purchasing the iPad...Steve Jobs, please take note!).
Okay, I can see what you're thinking, here I go fiddling with details. You have this great cover, you love it, you worked on it yourself for hours or with your trusted commissioned artist, and it's cost you already a lot of sweat and dough. And besides, where's the problem? You just reduce it with whatever technical program you can lay your hands on, it's click and go, right? Sure you can, but it's going to look awful. Crowded. Illegible. A meaningless blotch.
So what's the answer? Start thinking small. What does it require for a postage stamp image to draw attention? Good color, yes, something sharp that catches the eye. Primary colors are best - red, blue, yellow (no funny colors, in-between shades, they won't work in small size). But more than the color - remember a cover shows up in BLACK AND WHITE on the Kindle (and other e-readers) - what is important is the SHAPE and COMPOSITION (by that I mean the relationship between the letters of your title and name and what's around it). It's got to be simple, clean, striking. No jumbles, please! Your title has to stand out, either black letters on a clear background, or the reverse, white letters on dark. Yeah, otherwise it's not legible.
But that doesn't mean you should stick to simple geometric shapes. No, you can do whatever you like: you can go poetic, atmospheric, ghoulish, volcanic, whatever happens to have a "feel" that reflects the meaning of your book. Is it a thriller, with murder galore? Have dripping blood and a sharp knife! Is it a sweet sixteen YA romance? Have kissing lips or holding hands...Whatever, it's up to you but you get the idea! Good luck!