Not putting down the software, but I did look at the about page and didn't get much convincing. Also, even though I'm not mainly a PHP dev, the assertion that "ancient, legacy PHP/MySQL code bases" doesn't apply to bbPress or Vanilla (or at least I hope not, I haven't really looked at the code in a while).
Also, and this may be what sells it to me, can I use it comfortably if I'm disabled? I've built some discussion forums for sites that cater to people who have difficulty navigating cluttered environments (some have suffered strokes or other brain injuries) and some who are legally blind. Can they replace their forums with Discourse?
Other forums, even the crappy legacy code ones, still have real HTML links that can be browsed. Can I use it while having JS disabled (let's say I'm some privacy freak)?
Edit: Should have read the FAQ first ;) So my Lynx and text-to-speech users are out of luck.