I love logo design. We toured a corporate identity firm when I was in college, and I so very much wanted to work there! This the story of the Federal Express logo, and the hidden arrow therein. Some people see the arrow and some don't, but once you've had it pointed out you'll always see it thereafter. (I'm trying to use every word I know that begins with "there" today. So there.) The designer likens the arrow to a punchline, which must be delivered correctly.
The power of the hidden arrow is simply that it is a “hidden bonus.” It is a positive-reverse optical kind of thing: either you see it or you don’t. Importantly, not “getting the punch line” by not seeing the arrow, does not reduce the impact of the logo’s essential communication. The power of the logo and the FedEx marketing supporting the logo is strong enough to convey clearly FedEx brand positioning. On the other hand, if you do see the arrow, or someone points it out to you, you won’t forget it. I can’t tell you how many people have told me how much fun they have asking others “if they can spot ‘something’ in the logo.” To have filled in the arrow, or to somehow make it more “visible” would have been like Henny Youngman saying “Please take my wife” instead of “Take my wife. Please.” Punch lines that need to be explained are neither funny nor memorable.