CoffeeScript has significant whitespace.
So instant no. It makes it a pain in the butt to parse. It’s annoying. Most people (except for Python developers) don’t like it. Due to being difficult to parse, you will have problems when you try to refactor. Not to mention the problems of re-indenting everything when you have really long and ugly blocks you want to clean up (happens).
Stick with JS. More people are experienced with JavaScript afterall and critical mass in attracting developers is important for Discourse.