Where I used to work there was a quote on one of the doors from David Ogilvy, "When you advertise fire-extinguishers, open with the fire."
I always liked that quote because it's something often missed when people are trying to explain what they're selling or even just trying to convey. I see this happen the most on web sites for applications that have a strong visual component. I can never understand why someone who wrote an application that does X doesn't have a nice screenshot on the front page showing it do X.
This entry started because Jason pointed me towards a MovableType plug-in called BigPAPI that does something very cool and very visual (allows plug-ins to affect MT's interface), yet there are no screenshots to be found.