Key here is that McLuhan doesn't key on content, but rather on the medium itself. Not the medium. The message. In Understanding Media, with its subtitle "Extensions of Man," he offers up some of the key questions that continue to provide a key for understanding media.
Question 1: "What does it (the medium or technology) extend?" What in the human being is enhanced by the medium?
Question 2: "What does it make obsolete?" With this medium introduced, what does it "crowd out" or render unnecessary?
Question 3: "What is retrieved?" What element of humans is brought back to life as a result of the medium?
Question 4: "What does the technology reverse into if it is over-extended?" If the medium reaches too far, becomes too important, makes too big a footprint, then what do people do to retrieve what they lost?