In the comment thread for his last post, Tim provides my new preferred master metaphor for media:
Charlie Stross has a different metaphor. He compares the novel to the corset, and the novella to the bra. I love it! What could be more obvious, more taken-for-granted, than underwear? And yet, what strikes us as an odder set of conventions now than the underwear of the past? It’s made to fit and support our bodies, but it reshapes us too, as we wedge ourselves into it.
Seriously: I am thinking deeply on this. I think it might be really, really useful.