With the proliferation of “retina” displays icons with a low pixel density have become a real eye sore. While discourse is pretty awesome on a MacBook Pro retina the markdown editor icons standout as ugly and pixelated. Simply raising their resolution would go a long way towards improving the visual quality of the site. I think a similar undertaking is going on at Stack Exchange (if your using their version of markdown).
I know @codinghorror loves him some retina iPads.