My first impression of Zulip was "this feels like Slack".  It is
visually busier than Discourse, and I had a harder time
understanding context.  I had to step up the font twice to read
it.  I couldn't find how to hide the user list.  I felt lost. 
Based on my first impressions, I probably wouldn't engage via Zulip.

I had a negative first impression with Discourse, too, as "yet
another forum software", but it grew on me.  Dozens of UX
details were done just right, like "I wonder how I can do this
thing... oh, there it is!".  Or it could have been the Plone
community is engaging, or a wealth of information already
existed by the time I joined.