IMO its origins in the gaming community, and all the moderation features that grew out of it, make Discord a much better fit for open source communities than Slack. For example:
- Individuals can block and report other users
- There are tiered mod levels
- Per-community pseudonyms, but a single account makes it easier to track bad actors
Markdown support, including syntax highlighting, is actually better in Discord than Slack already, too.
If you haven't checked out an OSS community on Discord yet here are a few:
A framework like Phoenix would be awesome. Even something more lightweight (Express-like) would be pretty handy. Clojurescript front-end, Clojure backend...