Apart from the threading model, I love Zulip for its openness.
It's quite sad Slack and Discord are often the first choice for open communities. They are siloed and unless you are deliberately searching in a specific workspace, you'll never run into the information in a search engine.
In comparison, Zulip:
- provides HTML export functionality https://github.com/zulip/zulip-archive#zulip-html-archive (example https://leanprover-community.github.io/archive )
- URLs are nice and encode meaning (e.g. https://memex.zulipchat.com/#narrow/stream/279601-hpi/topic/...) ... compare with Slack/Discord meaningless character sequences
- I'm excited about https://github.com/zulip/zulip/issues/4817 , which would allow viewing workspaces in 'guest' mode, without registering at all