When writing out text in this way it is intuitive enough, and works nicely. However, when going back to make edits, it is not a fun experience.

OSes don't know quite what to do rendering cursors around the hidden markdown characters. Arrow key navigation is rough. Adding text at the beginning/end of a styled span is ambiguous whether additions should go inside/outside.

But for composing markdown separately and then displaying "rich" text in contexts that only support plain text -- this could maybe be interesting.

For adding more markdown support, headings seem like a useful and doable next feature. And tables would be a feat!