Eliezer has added details of how magic works, but I don't think he's intentionally changed anything from canon.
He has.
Eleizer has: changed the fundamental mechanism for the patronus, made dementors killable with patronus 2.0, edited the magic interactions between Harry and Voldemort to be dangerous to Harry as well as Voldemort, changed how the elder wand works (It's...maybe a +5 wand under normal use, and is only an "infinity+/-1" wand when fed blood sacrifices), and made Harry's invisibility cloak work against dementors-Cannon!Dumbledore (or...someone) warns Harry explicitly that invisibility cloaks do not work against dementors.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 94. The previous thread has passed 200 comments.
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: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22.
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: