Pro Has a built-in immune system from trolls, spammers and bad actors
Discourse has a simple user trust system that makes moderating the forum a lot easier. Users gain more permissions as they gain more trust, which limits the amount of damage spammers and trolls can do. Discourse co-founder Jeff Atwood also founded StackExchange, which is the gold standard for gamification/moderation systems so you can also expect Discourse to get better and better at moderation.
Pro Modern & polished UI/UX with lots of great smaller features
The best feature is its design: discourse is designed to aid in the creation of high quality online conversations. Flat discussions with well implemented quoting systems, expandable/collapsible replies, infinite scroll, clean UI and many more features come together to form a highly polished forum experience that makes legacy forums show their age.