A couple of communities I'm a part of have switched over to Discourse over the past couple of years, and I hate it.

It's just a mediocre forum, but with "modern web" designs/quirks. It does the whole single-page app thing, tons of wasted space/terrible information density, every single thread is displayed in one giant infinite-scrolling list (I've yet to see an instance with proper "subforums", just tags/categories), and it barely works without javascript (although at least it works).

My favorite modern forum software I've encountered is XenForo, although it's neither free nor open source.