From the story:
...and the whole time it had been right in front of him in every Potions class. Potions-Making didn't create magic, it preserved magic, that was why every potion needed at least one magical ingredient. And by following instructions like 'stir four times counterclockwise and once clockwise' - Harry had hypothesized - you were doing something like casting a small spell that reshaped the magic in the ingredients. (And unbound the physical form so that ingredients like porcupine quills dissolved smoothly into a drinkable liquid; Harry strongly suspected that a Muggle following exactly the same recipe would end up with nothing but a spiny mess.) That was what Potions-Making really was, the art of transforming existing magical essences. So you were a little tired after Potions class, but not much, because you weren't empowering the potions yourself, you were just reshaping magic that was already there. And that was why a second-year witch could brew Polyjuice, or at least get close.
[snip] Harry had stared at the recipes and their warnings, forming a second and stranger hypothesis. [snip] A potion spends that which is invested in the creation of its ingredients.
This leaves me with two possibilities:
1) Harry invested the energy himself in the potion. Instead of just using his magic to release the ingredients' potential, he poured in the required magic from his own cores.
2) Harry can now create potions from any non-magical substance as if it were a magical substance.
I believe option 1 is the correct one. First, Harry didn't play a part in the battle, probably because he was magically depleted. He's learned just as much dueling as Neville, and yet contributed nothing and died offscreen. Second, Harry wasn't rejoicing the next day and testing out a dozen different types of potions. He didn't act like someone who just uncovered a global victory condition or new branch of magic. Third, wizards would have discovered this if you could simply make potions without investing in magic of some kind. As the name of the chapter implies, Harry discovered a tradeoff, not a loophole.
The fact that the light was impossible to Finite suggests that Harry did tap the energy of the acorns. It's implied that the magical cost to the creator of making a potion is a minor cost to reshape the components. So, the potion taps the light stored in the acorns, and Harry's magic is tapped only to do the reshaping. Probably most magical potions use the magic of the magical ingredient to do most of the reshaping work, so the user only has to invest a tiny bit of magic, while a potion not involving any magical ingredients might require much more input...
(The HPMOR discussion thread after this one is here.)
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. There haven't been any chapters recently, but it looks like there are a bunch in the pipeline and the old thread is nearing 700 comments. The latest chapter as of 7th March 2012 is Ch. 77.
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.
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.
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: