This is the first Less Wrong post which I have read and really deeply agreed with. I haven't done much teaching, but I am used to teaching those who I am on equal footing with, my fellow students. Teaching people who don't have background knowledge, who might not feel comfortable saying 'I don't understand' is really hard.

From my inept experience learning to dance I can say it makes it much easier when the teacher recognises that it is hard for you, although easy for them. That little piece of recognition, even without the ability to modify your teaching style, makes learning easier.