Learnability

 Well, the essay emphasizes and uses examples of Learnable Programming, as a response to Khan Academy's (very clever) programming lessons, by Bret Victor who was credited with inspiring them, but it's really about learnability in general, and as he says,

The canonical work on designing programming systems for learning, and perhaps the greatest book ever written on learning in general, is Seymour Papert's "Mindstorms".
Designing a learning system without a solid understanding of the principles in this book is like designing a mechanical system without understanding "the lever". Or "gravity". If you are reading this essay (and I'm pretty sure you are!) then you need to read "Mindstorms".
Seriously. I mean it. If you are going to design anything whatsoever related to learning, then you literally need to read "Mindstorms".

Well, actually if you're right here then you're not reading that essay at all, you're reading a quote from that essay. But the advice may still apply.

Or then again, maybe not.