I would like to have the same WYSIWYG markdown editor as the one on Stack Exchange's websites.
The most wanted feature is the keyboard shortcuts like ctrl+k to add backticks around selection, ctrl+l to make links and so on. I'm pretty sure that will interest many people using github all the day long.
I saw the post from 2008 (yes, it's far now) from Jeff Atwood itself explaining that Stack Exchange websites use WMD editor.
As seen there, the project becomes pagedown, but I don't know how to use it in a browser (Firefox at least) for a known list of websites. (It's super easy for a owned web site to integrate the lib, but what about in a greasemonkey script to enable it where we want ? )
As a workaround usable on firefox, I can use the external editor stackedit to open the
textarea, but I prefer to have the editor directly in the web page. And as related there,
StackEdit connects to Google Analytics, Google Drive, Dropbox, Bit.ly, Wordpress, Tumblr and probably some other ones.