As a first-time user of Discourse, the yellow "Welcome to The Rust Programming Language Forum — thanks for starting a new conversation!" box was actively unhelpful. Could the contents be modified to mention that "Hide this panel with the button in the upper right to see the formatted preview of your post."?
I spent a while struggling to find a preview of my post until I found Make sure users know that a preview is being generated below panel - ux - Discourse Meta, which reveals that it's beneath the box. Once I knew what I was missing it was obvious how to close it, but my first assumption was that it's just making the content entry box narrower than it otherwise would be.
The blue "Your topic is similar to..." box has a similar appearance, but it doesn't completely cover what's underneath. For me that was a much more effective hint that there's useful stuff underneath it.