McLuhan's 'The medium is the message' has always been interpreted to mean discussion about the physical substrate. That allows people to talk about an electric light build as carrying a message. or to say things like 'the same content on television means something different than that content in a newspaper'. Etc.
But I thing there's another, more subtle, aspect to the slogan 'the medium is the message'. And that is this: that the 'meaning' of a message isn't the meaning of the words (say) contained in the message. That this content is the carrier for the message, which is (in a certain sense) subsymbolic. For example, when you say 'Get out of town' to a lawbreaker, you mean one thing, and when you say 'Get out of town' jokingly to a friend, you say something else. The 'message' - that is, the words 'Get out of town' - do not constitute the content of the message at all; the 'content' is actually the reaction produced in the receiver by the message (which is why an electric light bulb and a 300 page book can both be messages).
Now we can take this a step further (and this is what I think of 'the medium is the meaning'). The 'meaning' of the message, properly so-called, is constituted of the state of affairs described (referred to, represented by) the message. Thus, 'snow is white' means that snow is white. But this meaning is not the content of the message. You may be telling me that 'snow is white' but what you are actually saying depends on a wide range of factors - whether or not I had previously thought that snow was white, for example. On this view, again, you would think of the meaning as the carrier of the content..
But what is the message? It is a bit misleading to think of it as something that is actually 'carried'. Because, at best, it represents some intent on the part of the sender, and intent isn't something that can be tcarried in a message (it can be expressed in a message, but this is something very different). This is important because it breaks down the idea that there is some zone of shared meaning (or whatever it's called) between the two speakers. Even if there is a shared meaning, it's irrelevant, because the meaning is just the medium. It is simply the place where the interaction occurs. There is an interaction, but the interaction is not the transfer for some meaning. Rather, it is an attempt by a sender to express an intent - that is, to carry our some action (specifically, the action of causing (something like) a desired brain-state to occur in the listener).
The 'content', as McLuhan would say, is the receiver. More precisely, the content is the resulting brain state. The content is the change in belief, attitude, expression, etc., in the listener, that is a result of the transmitting of the message, the rest of the environment at the time, and the receiver's internal state. "What colour is the wall," asked the listener. You turn on the light bulb. "Ah, I see." he says.
This entire system is fraught with incompleteness and vagueness. The sender, for example, can only have a partial idea of the content he or she is actually sending with a message. There is the sender's intended content ('the wall colour is green') which - inescapably - becomes entwined with a host of associated and unassociated content when encoded into those words. Because the set of words 'the wall is green' is inevitably a crude abstraction of the actual mental state the sender wishes to reproduce in the listener. The encoding itself encodes, en passant, a raft of cultural and situational baggage. It exposes the sender as an English speaker, who uses the system of six primary colours, who is referring to a terrestrial object (otherwise, it would be the 'bulkhead'), etc. The tone of voice, handwriting, etc., can contain a multitude. And the like. The actual transmission can best be seen only as a scrap - the barest hint, which will allow the receiver to build a complex mental picture, one which presumably accords with the one the sender had hoped to create.
The received receives the sentence 'the wall is green' and decodes the 'meaning' of the sentence, which is a reference to a colour of a wall. This may or may not have been accompanied by some sensory experience or action (the turning on of a light bulb, say). These all, depending on all the other factors, cause a new mental state to emerge in the user's mind. It may even be accompanied by some internal perceptions (such as mental talking to oneself). The receiver may think, on hearing the sentence, "he thinks I'm stupid." It should be clear that the 'content' of the message, as received, may have little to do with the content of the message as sent. Moreover, the sender knows this. The sender may intentionally cause the receiver to receive the insult. The expression of the intent may be semantically unrelated to the intent itself (just as the swinging of a bat is semantically unrelated to the hitting of a home run - it is only when viewed from a particular perspective that one can conjoin the one as an expression of the intent to do the other).
This isn't unique, of course. J.L. Austin spoke of 'speech acts' decades ago. John Searle talks about 'indirect' or 'illocutionary' speech acts. Max Weber talks about 'sense' and 'intention'. Wittgenstein's doctrine that 'meaning is use' could be considered an 'action theory of language'. Habermas talks about language as the vehicle for social action.
And there may not be any specific intent (not even of externality) in the sender's mind. "He talks just to hear the sound of his own voice." A lot of communication is just verbal flatulence. It nonetheless has content, because it nonetheless has an effect on the listener (however minimal). The actual effect may have little, if anything, to do with the intended effect. Semantics is distinct from cause; the sender's intention does not have causal powers, only his or her actions do (and intention underdetermines action, and action under-expresses intention). That said, we are sensitive as listeners to this intention, and have a means (mirror neurons, for example) of perceiving it.
Language is the vehicle we use to extend ourselves into the world. It is what we use to express our intent, and hence to manifest our thoughts as external realities.