Just a reminder that Brett Terpstra’s Markdown Service Tools are very handy if, like me, you prefer the contextual menu over something like Keyboard Maestro.
I agree with @BLUEFROG that it’s mostly both quicker and more elegant to type the actual Markdown code than to mess around with scripts and shortcuts. But today I pasted some plain text that I then wanted to turn into a long Markdown list. The appropriate Service made this easy, and the collection contains many other handy tools for wrapping, unwrapping, smart quotes and so on.