All 20 rules are important, the first one maybe even more!
Albeit in the book the rule is more about the design here I use it to take a more general approach. When you communicate you must have something to say. A message, and idea or a concept you want to get across. It sounds self-evident but sometimes is not the case.
If you are examining your visual aids for a presentation you should evaluate a slide for the meaning it has. Is it coherent with the presentation? If not ditch it, no matter how beautiful it looks. At the best case is useless, at the worst is distracting from your main message.