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:
You do not need to rot13 anything about HP:MoR or the original Harry Potter series unless you are posting insider information from Eliezer Yudkowsky which is not supposed to be publicly available (which includes public statements by Eliezer that have been retracted).
If there is evidence for X in MOR and/or canon then it's fine to post about X without rot13, even if you also have heard privately from Eliezer that X is true. But you should not post that "Eliezer said X is true" unless you use rot13.
Good point: you should treat your own transfigurations like toxin - particularly if you are transfiguring from a toxic material or to a non solid. If the armor was imbedded in you and turned back to a non-toxic solid (wood, stone) that wouldn't be much worse than steel. I guess you might as well transfigure it out of ice if possible. In this example I think that not transfiguring the armor because you are afraid of transfiguration (a reasonable fear to instill in children, but not adults), and therefore taking a blasting curse to your unarmored chest is worse.
Partially it depends on the difference in size between the base materials and the target form. Things can be Transfigured smaller (Harry's father's rock to tiny diamond) or bigger (ice cube to rocket). The rock reverting could tear Harry's hand off, conceivably- shards of metal (or rust) getting significantly bigger or smaller once inside your body seems likely to be uncomfortable.