I agree also that Tom has a point, but at the same time, nobody HAS to use Markdown to start participating. In this sense there is no barrier to entry. And all the buttons are there in the editor if needs be.
I’m not sure that the basics of markdown are even complicated enough to merit a tutorial. I’d have thought a simple explanatory text, containing the main aspects of markdown, with source on the left, result on the right, would be ample. Then a link to ‘advanced’ and you’re done.