The announcement by project lead, Jeff Atwood, sold me. This is one rather incredible guy, a game changer of sorts, most notable for helping create what is likely the most important programming resource on the web. So whenever he goes and creates something else, I naturally do pay attention, and this Discourse thing looks amazing, from by limited usage so far.
I am just jealous that it's written in Ruby, and not Python. I am also jealous that the main service runs on Ubuntu, and not Debian.