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.
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.
Vanilla Porter can import posts (comments), threads (discussions), users, categories, roles and passwords (hashed) from most other forum software: bbPress, Drupal, MyBB, NoteBB, phpBB, PunBB, SimpleMachines 1&2, Toast, UserVoice, vBulletin, Xenforo, YAF.NET and others. Some importers support avatars, attachments, signatures, and PMs. A few support tags and user notes.
Imports aren't flawless, but they do most of the work.
It is difficult to customise Vanilla in a way where it can be source controlled. This may be a benefit for communities who just want to get up and running, but is a drawback for developers.
Old versions of a post are only visible to admins, which may lead to a credibility problem. The amount of time during which users can edit their posts is configurable from a few minutes to indefinitely.