HN does use Markdown...it's just a very incomplete implementation. Since HN is written in Arc, there would have been no off-the-shelf parser, and I assume pg opted to use the simplest subset that would work. There also seems to be an aversion to fancy-pants stuff on HN, so it may be intentional. Why get clever when clarity isn't improved by doing so? (That said, I kinda miss having full Markdown link support.)