Discourse is a Civilized Discourse Construction Kit, a free, flexible, fundamental building block for online discussion communities of any size, tiny to huge.
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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.
Pro 100% free and fully open source
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 Helpful community
Pro Good to go out of the box yet infinitely tweakable
Discourse has sane, safe out of the box defaults, but a million dials and knobs to tweak.
Pro Hosted and self-hosted solutions
You can host it yourself in a $5 cloud server or on any Linux server you already have. And if you don't want to deal with this, you can use the hosting platform from the development team.
Pro Live preview while editing the post
The post editor has a synchronized side-by-side preview of the Markdown rendering of the post.
Pro Easy one-click upgrade
Discourse can upgrade itself with one click in the admin panel. Not the same can be said about MyBB, or most other forums.
Pro Effective search
Pro Export your posts
Pro Official Akismet Anti-Spam Plugin
Runs in modern browsers and works identically on desktop and tablet and smartphone without the need for a native app.
Pro Discussions can be organized in categories and tagged
Discussions can be tagged with an arbitrary number of tags, separately from categories. Categories are hierarchical and don't scale; tags do.
Pro User selectable themes
Users can choose between the themes made avaliable on their instances, and even use different themes on different devices (mobile vs desktop, for example).
Pro Extensible via plugins
Con Complicated setup
Compared to standard PHP+MySQL apps, Discourse is more complicated - the app lives in a Docker container. However, a Docker image is available, which reduces the setup time to about 30 minutes.
Con Bad noscript support
When using a Discourse forum without scripts the experience is greatly reduced. It's really hard to use and is read-only, meaning that one can't comment or create threads without scripts.