If you let me butt myself here, and apologies for a long-ish post—I gave an informal talk about this a few weeks ago to a group of friends, so I thought I share what I gathered during my research.

For me, the best thing I've read about media theory is No Sense of Place (here's a short summary that I believe focuses on the key points of the book). The author focuses not only on how different ways of communicating (the medium, from McLuhan) affect us, but how it relates to physical space.

An great example in the book focuses on how you would tell your parents, friends or university professors about your summer vacation: each will get a different version: the "clean", the "adventurous", or the "cultural" one. And that works because each group is in a different "physical" place. If instead you got a welcome party, where the three groups collide, you'd be "forced" to tell a safer version to not make it awkward for anyone in particular. Yet this version is a weird compromise of the three versions you'd tell otherwise. If you liken this analogy to, say, Twitter, or any social network, you get this "welcome party" setting all the time: you're constantly evaluating how different sections of your audience are going to react to what you say, or you'll get misinterpreted by any of them (danah boyd calls this "context collapse")

Coming back to a more concrete explanation of "medium is the message", in my opinion, McLuhan tends to be obscure on purpose, he will mention something, and then you'll need to interpret it through your own preconceptions. In my opinion, this makes it so each person takes away something different from what he has to say. He gave a long interview to Playboy in 1969, where the interviewer forces him to be more concrete. I got a lot of interesting quotes from that one, here's one:

Q: how [can you] hold the development of printing responsible for such apparently unrelated phenomena as nationalism and industrialism?

A: Nationalism didn't exist in Europe until the Renaissance. The printing press, by spreading mass-produced books and printed matter across Europe, turned the vernacular regional languages of the day into uniform closed systems of national languages – just another variant of what we call mass media – and gave birth to the entire concept of nationalism.

The homogeneity of money, markets and transport also became possible for the first time, thus creating economic as well as political unity and triggering all the dynamic centralizing energies of contemporary nationalism. By creating a speed of information movement unthinkable before printing, the Gutenberg revolution thus produced a new type of visual centralized national entity that was gradually merged with commercial expansion until Europe was a network of states.