Zulip is not a Slack clone. Instead,
Zulip is an open source alternative to Slack which has
- Better Markdown support
- Julia Syntax highlighting
- Unlimited message history for us and other open source communities
- Beautiful built in LaTeX support for displaying math
- An approach to conversation threads that may be initially a bit different from how Slack handles it, but I think much better once you use it a bit
- Really responsive and helpful developers. If you have a problem or complaint with Zulip you can open an issue on their Git repo and expect a quick response! This really contrasted with my experience with Slack at various times, especially when Slack started screwing with text formatting.
(But there is also a time-limited “ephemeral” channel for some informal chats, to emulate the slack experience.) Discourse is nice for a forum feel, but zulip is nice for a more lightweight feel while retaining the powerful features of a discussion system. (Just for an illustrative example, many more messages fit on a Zulip screen than on a Discourse screen.)
I really like Zulip, and a bunch of our most distinguished community members have moved their contributions from Slack to Zulip because they value the accessibility, threading, searchability, formatting, open-source community, data ownership, and so on. I hope that trend continues.