I disagree with the assertion that merely providing the impetus for a forum post means that it’s encountered very often. I can easily imagine that @mgenev encountered one or two tutorials on subjects he was very interested in learning about that Coffeescript made it difficult to understand.
Regardless, I suppose this really just boils down to the ubiquity of Coffeescript, which I know of no good way to measure. Because despite Coffeescript being a very shallow one-to-one translation, I still personally find that it makes code more more difficult to understand (which is obviously due to the fact that I don’t use it very often). If most developers were indeed more comfortable with Coffeescript than Javascript, then it may in fact be a good language to use in tutorials.
I do think it’s important to note however, that accepting Ember.Object.extend is necessary to understand Ember (at least as a beginner), whereas Coffeescript is certainly not.