Attention is a function of the medium

The best thing I've read for a while: in response to a question from Tyler at Marginal Revolution about what's wrong with long books, Michael Blowhard writes:

"when you think about it, isn't it incredibly ... audacious or arrogant or something for authors to ask us to read (for instance) 600 pages?...what individual - and whose individual voice - merits that kind of attention? Would you voluntarily say, OK, I'm going to listen to Person X yak on for 10 hours straight? I mean, would you do that often?"

Or perhaps in other words - your choice of medium is a crucial influence on how much attention you can command for your message. Surely it isn't just that it takes a long time to write a book that ensures people will spend a long time reading it - rock albums, operas and films all take as much time to create as a book but far less time to consume. As bloggers we may have committed a grave strategic error - if we spun our points out to 200 pages with the customary filler, photos and charts not only could we sell them at £10 a copy but people would apparently commit hours of their lives to reading them.