On the specific issue of Markdown syntax highlighting (which would affect the edit window, rather than the preview window), Gruber has - just today - spoken:
I created Markdown to use in BBEdit without any syntax coloring at all, and to this day, I do most of my Markdown editing in MarsEdit, which doesn’t color or style Markdown syntax at all. (It should though! Markdown is even better with some syntax coloring and styling.)