1. Client-side performance. Little JavaScript and lean CSS. Even the KaTeX math plugin renders math server-side.
2. Clean but discoverable UI. The focus is content. Not a single dropdown nor a hamburger menu.
3. Extensible, but critical features are built-in to provide a foundation for plugins. Features such as notifications, moderation, permissions system, and full text search.
4. Pleasant on mobile for both browsing and content creation. This means a textarea instead of a rich text editor (by default).
5. Everything degrades gracefully when JavaScript is off. For example, preview-as-you-type becomes a "Preview" button if JavaScript is disabled.
6. The relational database for everything, including full text search. Data integrity is easy, no out-of-sync elasticsearch databases here. Runs on Postgres, MySQL, and SQLite.
Boring technology => easy to deploy and run in production.
The codebase is written in Ruby, has slowly developed over the years, and has extensive test coverage.
I am very curious to hear what you think, and whether you've ever felt a need for a forum like that.