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.
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.