Perhaps sacrifices are the real source of magic. Not really equivalent exchange, given the trivial uses magic is usually put to, but that's thermodynamics for you - 'you can't win, you can't break even, and you can't quit'.
I'm immediately reminded of discworld where technical improvements in magical theory have gotten to the point where a spell that originally required the sacrifice of a human being can now be performed using a few ccs of mouse blood.
Hmmm, what if the practice of magic is weaker in the present of MoR because ritually sacrificing a few dozen peasants for purely experimental ends is considered in bad taste?
I can see Dumbledore BSODing over the discovery that Hogwarts is actually powered by the hearts of ten thousand orphans somewhere down in the foundations.
Update: Discussion has moved on to a new thread.
The hiatus is over with today's publication of chapter 73, and the previous thread is approaching the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it.
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. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
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: