We gave Zulip (and Mattermost, Element/Matrix, Rocket Chat etc) a try when our company was looking for self hosted alternatives to Slack. Zulip seemed like technically the most polished product. Smooth and easy get-running experience (Element was the polar opposite). Lightweight UI that doesn't feel buggy. I'd have liked to see a bit bigger font but what ended up being the showstopper was that new users were very confused about how to write in a channel (or stream how they call it). Everything needs to be in a second level "topic". In pretty much every other chat app one can just select a channel on the left and get a text input field where you can write your message. In Zulip though you don't get the text input. You have to either create a completely new topic which new users really don't understand or click on any of the past messages in the stream to create a reply to that topic (but not that specific message). I suppose it works well after you understand the messaging model but it represents such a high initial point of friction that we had to give up on it. Non-technical users had a really hard time and the switch from Slack was not high enough on the priority list to push through with it.

tabbot: if you happen to see this, please please consider some kind of easy-mode which:

  - by default always shows the text input no matter if you clicked on a stream or topic
  - doesn't put the cursor in the "topic" input but the actual message input
  - allows sending a message by pressing enter
  - scrolls the window down when sending a message
  - scrolls the window all the way down when clicking on a stream
  - has a slightly bigger font size
  - maybe also some simplification of the UI. You don't need the name of the stream above every message (see https://i.imgur.com/oeBY5yk.png )

I'm convinced with a bit of work to make it easier for new users you'll have a big boost in growth.