In the context of this post, an important thing to add is that threads in Zulip also have just a single level of hierarchy - it's not at all like the tree structure here on HN, or on Reddit. We've felt that single level is the right balance for having a conversation that someone else can come to an hour later and easily read and follow.

The way it works in Zulip is quite different from Slack, though. Here's a description in another comment in this thread: https://news.ycombinator.com/item?id=23863588