But you do now know about it. People telling each other about a hidden feature (via HN or whatever) is a common and valid way for non-essential features to be discovered.
If you designed UIs assuming that that sort of user-to-user education doesn't exist, you'd be forced occasionally to reject a potentially handy power-user feature on the basis that there's no good place to promote it without cluttering the design. Which would be a shame.
Considering that "m" is half the known keyboard shortcuts (I'm ignoring ? as it represents a recursion I'm unwilling to face right now), that is odd indeed.
I love the idea of treating web-sites more like applications. From a programmer's point of view - yes, keyboard should be a more utilized way of interacting with a web-site, beyond arrow-navigation and filling text-boxes.
