Thanks for the feedback. I'll be playing with the idea and iterating on it until it's perfect or until I'm sick of thinking about it -- neither likely to happen any time soon.

I hadn't seen the side-by-side editor in Ghost before; this plugin was more immediately influenced by Markdown editors like MarkdownPad and Mou.

Your four checklist points are all reasonable improvements, and they're most likely to be implemented in reverse order.