having followed this, the recent, sensible (to my mind) posts help
in a WB skin, there are a few basic points that *I* find essential:
* fun title bar, i see them a lot

aquarium is good for this
* reasonably thin borders, i run at a low res
* borders that show up well on black, a must.
i spend most of my work time using terminal emulators to work on unix, so all there is for WB to skin is the title bar and borders. there aren't even scroll bars.
scroll bars are important in notepad, etc
most of the other frilly bits in skins i don't tend to see (black windows don't have a lot of controls

).
however, there aren't really "average" requirements, so the majority of skins don't seem to be designed with this in mind
the one thing i would ask for in more skins though is more "interesting" titlebars. they are so obvious, at least to me.
skins i like for this are: Bleu, Aquarium, devoir, and (perhaps strangely) XP Ocean.