I’ve been looking at…

I've been looking at illustrated covers in particular, since so much of genre fiction cries out for an illustrated cover (rather than sticking to text, lines, abstract shapes, etc.), trying to figure out what looks sharp and professional. Here are some techniques that seem to make illustrated covers look better:

1) some book covers put the illustration in a separate box, which helps it look very clean and somewhat modern. Like these:

Sabriel: http://pinterest.com/pin/217369119485667351/
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?