One of the design goals of Discourse was that it should work well on mobile phones. I guess most other forum software is either from the time of before widespread smartphone use or it doesn't consider mobile users. With that being said, I actually don't like discourse's UI and prefer more classical forums like PHPbb.
Working well on crappy toy devices = working shittily on actual computers
Smart watches should have taken off, so everything could have been made post stamp-sized to work well on them and become completely unusable on a screen larger than your hand.
Discourse goes a bit overboard with the javascript and all the bells and whistles but I don't understand how anybody could prefer PHPbb over it, other than familiarity. That being said I always found PHPbb abysmal to use, even in the early 2000, so clearly I'm biased.
My main issue with Discourse is that I prefer HN/Reddit-like threading for replies rather than linear comments, but PHPbb does the same and there are pros and cons for both formats anyway.
