I have shared this privately, but I think it is important to share publicly. I am extremely excited about this change, it is transformative to Discourse and opens the door for an enormous amount of communities 
This one got me yesterday, might as well post here.
Once you get into a “HEADING” state, there is no way to easily undo it. Eg type ### text. This is particularly problematic if you kick of a post with a heading and change your mind, you need to flick back to classic composer to undo that.
I wonder if we need a toolbar item for this, that lets you flick a line to “heading 1/2/3 - paragraph” etc.