All the communication technologies we use -- telephones, newspapers, radio, IM, e-mail, mailing lists, TV, books -- are mired in historical cruft that keeps us from seeing clearly what to build next. It is useful to go to first principles, then reexamine whatever communication task you have at hand. So let me suggest the following basic dimensions of communication: timing, audience, mode, length, persistence, production level, identity and permission. The list with explanations and more on my wiki here.