Isn't the point of Markdown that it's self-documenting and editable in any text editor? If I were to write an editor for Markdown (or, more likely, a plugin for Vim or Emacs), I would focus on providing editing shortcuts and autocompletion. WYSIWYG editing is more useful in standard HTML than it is in Markdown.
It is, but the point of the project wasn't to create yet-another-markdown-editor, it was so I could play around with WebRTC and get two browsers talking to each other directly. I used Markdown just so documents wouldn't be plain text.