I digress on the category definition. Public messaging (without PM or DM features) is still messaging; and often far more useful than trying to forward 1:1 messages in order to bring additional participants onboard.
It's worth noting that GH/BB/GL have all foregone PM features; probably for the better in terms of productivity: messaging @all is likely more productive.
Related discussion: I've built FastComments to be "realtime" and support markdown. Been thinking of reusing the infrastructure to launch a little chat app, but not sure what the killer feature would be aside from being lightweight...
WhatsApp isn't "markdown", but supports code blocks using triple backquote (and uses underscore for italics and star for bold, which I think is a lot better than markdown anyway).
