Third-party Markdown plugins have existed for years, but now that Markdown will soon become a (semi-) native feature, it will reach a broader audience than ever before. This is great news. Markdown is beloved by the tech-oriented crowd that forms its base today, but non-techies should have reason to love it even more. I think it’s much easier to pick up than HTML, and harder to screw up.
This also makes me think it might be a good idea to package jQuery.sidenotes, which deals with Markdown content, as a WordPress plugin.