> One issue: Some users get confused over streams and topics, often posting anything in whatever was the last topic they accessed rather than going to the correct topic or starting a new one

This has been a problem ever since email: many people reply to the last mail they have from a list, sometimes changing the subject, and then get surprised when the mail is threaded under the mail they replied to.

I've seen this occasionally on Zulip instances, but a reminder and a suggestion that the topics are like email thread subjects or forum thread subjects tends to help.