I think most people are missing the point of what Jeff’s trying to do. He’s not trying to expand Markdown or add features. He’s trying to unify it. Github, SO, Discourse and any number of sites use their own interpretations of the Markdown spec. This means that any Markdown written on Github may or may not work on Discourse, and vice versa. What he and the team are trying to do is create a more concrete, unambiguous spec that all sites can use so we have consistent implementations of Markdown across the internet.
I see nothing but good intentions here, and also nothing but good things can come out of this.