I am a huge fan of a “Discourse walk through” bot.
It can guide you through using Discourse in a single topic.
“Try uploading a picture”
“Good”
“Now try bolding some words in a post you make”
“Good”
“Now try quoting something I said”
“Good”
Etc.
It could all be done in a private message to avoid cluttering the general view of things.
I think it can even be more effective than Markdown Reference cause it is far more “fun” and can be a lot less strict.