In logo design, I've always been a fan of negative space because it has the potential to surprise you when you look at it. Or give you a completely new perspective and afterwards you can never see the image differently again.

Negative space can make the visible out of the invisible, but only if you can see the visible in the invisible. It is like a colour of its own that imposes itself on recognition and then remains forever.

One of my favourite examples is the Fedex logo. For years, for me, it was just five letters in two crudely mixed colours, until I suddenly saw the white arrow between the E and the little x. And since then I only see the white arrow and I am fascinated how it could, just like that, quite obviously "hide" between the two letters.