Seth Godin posted this graph earlier today:

Godin writes:

On the horizontal, they rank from asynchronous (meaning the creator and the responder are separated in time--like a letter) and synchronous (meaning the creator and the responder are in real time proximity to each other--like a phone call).

Up and down, I've charted the quality of the medium. Quality in terms of density of information exchanged. The 140 characters in Twitter is about as low density as you can get other than a stop light. A movie, on the other hand, is loud and bright and two hours long and there's audience reaction and it is edited and designed to evoke a response.

It very neatly shows the relationship between a medium's power and how interactive it is.  Godin explains more here.