Very nice article coming from the two of you. I'm glad to see you brought up Postie and Jetpack as I was really curious about those two myself. Seems there's room for all three.

My personal Most Wanted for post-by-email would be bbPress support. Not only would it be a really powerful feature (makes it a much more appealing platform for the mailinglist traditionalists, and it's also a great alternative route to mobile support), but it would also be nice to see one of the sister projects getting some contributor-love for a change.