I like a cover that shows people or creatures in action. Covers that are "designs" or made to look like the cover of an old book just don't do it for me. The cover should give me a big, pretty gulp of what's inside -- the tone, the scope, the content, the genre, the style. It shouldn't be generic or re-used art, no matter how good it is. If you put a picture of a sexy drow lady holding a skull on the cover, that book better be about drow necromancers.
That said, if I get a chance to look at the cover and the interior, the layout and interior art will have just about as big an impact on me as the cover (i.e. some but not a lot).
Examples of covers I liked:
Hollowfaust
FR: Lords of Darkness, Silver Marches, Magic of Faerun
Lord of the Iron Fortress
A lot of the covers and full-page illustrations in Dungeon Magazine (for instance, this month just you try to tell me that the picture of the mohrg by Jason Engle doesn't make you sit up and take notice)