1) some book covers put the illustration in a separate box, which helps it look very clean and somewhat modern. Like these:
The Lemony Snicket series: http://pinterest.com/pin/217369119485667357/
Fortune's Boats: http://pinterest.com/pin/217369119485648584/
2) some that are more scenic fill the page with the scene, so it feels like you're peeking into another world, especially for SF/F:
The Hobbit: http://pinterest.com/pin/217369119485667285/
The Lightning Thief: http://pinterest.com/pin/217369119485667323/
The Lord of the Rings: http://pinterest.com/pin/217369119485667299/
I like both those approaches a lot. I also strongly prefer covers that limit the palette to 3-4 colors; they just look a lot cleaner. What I don't like is a cover that just has an illustration sort of plopped onto a big empty page, especially a big white page.
What do you think? Agree / disagree? Other thoughts on what makes a great illustrated book cover?