I'd like to point out something awesome Eliezer did in the previous chapter, "Cheating". In canon, Potions as a discipline is hardly taught at all. The only thing you ever see Snape do in the books is give a list of ingredients and instructions, tell the class how long the class period is, and assign papers at the end of the class. This is one example of how J.K. Rowling wasn't really invested in developing the mythology of the universe, except as strictly necessary to make her plots happen. (There's nothing wrong with that; they're children's books, not "real" fantasy for adults.)
With the "Cheating" chapter, rather than trying to create a whole framework of Potions rules to understand as he's done elsewhere, he simply added a darn good explanation that legitimizes everything Rowling already showed us. When Hermione lectures Harry on "understanding the principles" in Half-Blood Prince, instead of scoffing about how there's never been evidence of any principles to learn, we can now imagine that there's a very good reason why Harry is never taught the principles of potion-making: if you're not smart, thoughtful, and careful enough to figure them out for yourself, you have absolutely no business knowing them at all.
When compared side-by-side as if they're in the same genre and directed toward the same audience, Methods of Rationality often makes the books look worse than they used to look. But in this case, future readings of the books will be made a little bit better. I wonder whether there are other ways that's true that I haven't noticed yet.
Seconded. In retrospect, the Canon teaching of potions now seems incredibly practical. The vast majority of students not only don't need to learn the theory, but it's a negative for them to do so. They only need to learn two things: 1) how to make the potions they'll use every day and 2) whether or not they even can make those potions or if they should just buy them from someone else. Constant repetition with minimal instruction is exactly what you need for a class that's more akin to cooking instead of calculus.
EDIT: New discussion thread here.
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. With two chapters recently the previous thread has very quickly reached 500 comments. The latest chapter as of 17th March 2012 is Ch. 79.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author's Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven, eight, nine, ten.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: