Given what I can do in notepad, this doesn't quite do it for me. Yes, notepad is not what-you-see-is-what-you-get, but it doesn't take that much to create a HTML repel and that followed by alt-tab allows me pretty much every thing I want. I am uncomfortable with 'some one else's idea of a good time' when it comes to feature selection---no <hr/>? Etc. Good idea in general, just not for me, not yet.
For a lot of users, WYSIWYG is good, especially if it really integrates with the web page you're trying to edit (not a non-styled IFRAME like in the traditional CMS editors).
I also usually write in Markdown, but the latest breed of editors is starting to be good enough that I may be able to swap. And as for <hr />, that is usually configurable in the editor setup.
Disclaimer: I'm an author of one the new editors, Hallo.
I don't think anybody has written that plugin yet. Could you post a ticket? I'm spending some days on Hallo and Create next week, and so could possibly write one.