The main differentiator of Zulip IMO is the threading model. Instead of chat rooms with multiple un-separated concurrent conversations taking place, every messa

The main differentiator of Zulip IMO is the threading model. Instead of chat rooms with multiple un-separated concurrent conversations taking place, every message* must start a topic or be placed in one (topics are like the subject line in an email). You can still look at the room and see all of the topics at once, but you can also focus in on a single topic. And it remembers your read progress at a per-topic level, so it's quite feasible to catch up on chats asynchronously. Basically Zulip brings the best things about mailing lists to chat. *: you can now turn off the topic requirement, in which case messages go into a "no topic" topic. But not using topics kind of defeats the purpose of using Zulip IMO.