The thing I like about White Wolf titles, and that is simultaneously frustrating, is that they have taken flavor text to a whole 'nother level. In essence, they have replaced background material almost entirely with flavor text. For example, the Aberrant book (both D20 and Storyteller) is half, HALF! flavor text with absolutely no rules or hints of rules until you are halfway through the book. And their flavor text is not that neutral, encylopedic tone of most RPGs. No, White Wolf has short stories, simulated news clippings, Web sites, personal letters, etc., etc., that slowly illustrate the tone and style they expect your game to take.
And for those unfamiliar (although I suspect it is just Craig and I reading this thread

) the World of Darkness is not just a "let's play the bad guys" sort of world. The PCs, at least in Vampire which I am most familiar with, are supposed to be these morally conflicted, angsty monsters, trying to maintain some semblance of their humanity while living their monstrous lives.
From a professional point of view, I find it most interesting how they work religion into the whole thing. Especially in Demon, where the characters are, of course, fallen angels living in mortal shells. This is territory I think most RPGs are reluctant to walk through.