That’s a lot of good stuff! But I disagree completely about the subject line being the only message.

When I’m in high-volume email mode, I’m reading and responding to one message after another; there’s no clicking to view the body. I focus on the message first, and the blank message body is jarring. I agree that the subject line should be a concise summary, so that scanning a list or searching will work well. But if somebody can’t take the time to send me an entire sentence, I find it harder to work up the energy to give them a proper response.