Two things: In Zulip, you can't not have threads. Threads in Slack are an afterthought and the UI actively tries to prevent you from using them (they're a narrow column on the sidebar and you can't see everything), whereas in Zulip they're a first-class citizen.

The second thing is that Zulip's UX is fantastic. You can zoom in and out of conversations with a single keystroke, and the UI is responsive and does exactly what you want. Slack is so slow on my new Ryzen that I frequently out-type it, and half my keystrokes end up interpreted as channel-switching keystrokes, so I end up having typed something random in a random channel instead. Fuck Slack, sincerely.

Slack's thread UI is simply atrocious. Zulip's UI makes it harder to not be part of an existing thread. In slack, anyone can just wander into room and start typing things that should be in a thread, outside of any thread, even (especially) if the thread they want is right there. That's not impossible with Zulip, just harder enough to do that it's not an issue in the same way.

If I understand Zulip's threading correctly (I can't log in right now to check), it imposes a strict two-level threading system. Quill (https://quill.chat) has nested channels but unlike Zulip, clicking on the "outer" channels doesn't show you all the content of all the "innner" channels.