Idea: Add buttons to the top (optional) that LOOK like a WYSIWYG (something they are familiar with), that simply apply markdown around text.
I think you'd see such much wider adoption with that addition, which is something I'd love to happen, because WYSIWYG's do indeed suck.
• a toolbar-like row of pulldowns: one for 'range/inline' markup (like bolding), one for 'block/paragraph' markup (like headers, blockquote, code-indent), and one for complicated inserts (like link+linktext, image+alt)
• each pulldown would be filled with side-by-side entries showing both markup and effect, like the best cheatsheet/reference pages are laid out
• selecting a bit of markup would do 'the right thing' with the existing insertion point or selection-range
Thus a user might momentarily click then dismiss the dropdown just to refresh their memory of the syntax, or actually pick from the dropdown for assisted inserts. (If they're regularly picking items, it could reorder by 'frecency' for convenience. If they're usually just looking, it might instead order by some form of 'infrecency'  – don't bother showing familiar constructs that have been recently typed.)
 At the moment while there are thousands of Google hits for ["frecency"] there are none for ["infrecency"]. It's an infrecently needed concept.
I wonder how you could add such help on top of a Markdown editor.
An alternative (albeit a more resource intensive one) would be to make ajax requests back to the server and have it perform the transformation and return it to the page. Unfortunately, that's probably not going to be satisfactory for realtime previewing.