Wise choice @eviltrout.
I prefer CoffeeScript, personally, its the only language I write in that I don’t think about the language.
However, Discourse is going to (hopefully) be around for a very long time. There are so many things that are going to be difficult for CoffeeScript in the future. For example, default arguments. null in CoffeeScript will defer to the default while in ES6 it will not because null !== undefined. I can’t see how it will target ES6 successfully and keep backwards compat.
Worth noting, CoffeeScript fell off the top 10 list on github. CoffeeScripters usually still like and write JavaScript, including the language’s creator. Discourse will get more contributors with JavaScript without question.
Better create a style guide first.