He explores the increasing complexity as we move up in the frequency spectrum to allocate more and more traffic. It is a great introduction for any newcomer who wants to get a global idea of what the different radio bands do in practical terms.
To close the circle, the article includes a detailed description of the different RF Explorer models, and how you can use this device to detect signals in the radio spectrum. Or, to paraphrase his words, a tool that will help you on "drinking the radio soup".
An online version of the article is available on Paul's blog here.