Just started evaluating MarsEdit, and so far I'm loving it.
A simple popup window with a Markdown format synopsis would be very handy for those of us who haven't internalized the format yet. The way Basecamp does it, providing a short and a long version, would be nice. Perhaps you could also embed an URL to the Markdown Dingus.
Maybe a "Formatting Hints" drawer that gets filled with appropriate hints based on what format you're currently using. Textile needs this even more than Markdown :-)
I'll think about it - the drawer might have to come out of the preview window instead of the main window, because that's the only place you currently select a particular formatter.
Maybe the preview panel could have a "Formatting Hints" button that pops up a small window with the appropriate hints. The important thing is to have the hints visually close to the editing window, for workflow efficiency.