Update: Discussion has moved on to a new thread.
The load more comments links are getting annoying (at least if you're not logged in), so it's time for a new Harry Potter and the Methods of Rationality discussion thread. We're also approaching the traditional 500-comment mark, but I think that hidden comments provide more appropriate joints to carve these threads at. So as of chapter 67, this is the place to share your thoughts about Eliezer Yudkowsky's Harry Potter fanfic.
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. The fanfiction.net author page is the central author-controlled HPMOR clearinghouse with links to the RSS feed, pdf version, TV Tropes pages, fan art, and more, 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:
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Eliezer Yudkowsky which is not supposed to be publicly available (which includes public statements by Eliezer that have been retracted).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.
In canon, the hardness and thickness of materials are described as stopping spells, especially stunning spells. Hagrid, e.g., is able to resist several Aurors' stunning spells for a few minutes because of his thick, hard, half-giant hide. No form of cloth or wool clothing is ever described as stopping a magical attack, but Harry can hide behind (presumably granite) gravestones for some time while Death Eaters blast away at them. Toilets, which presumably are not quite as thick or hard as gravestones, are shown as stopping one offensive spell but then exploding.
IMHO wearing metal armor is a brilliantly canonic tactic. The least plausible facet of it is that first years in January, average age 11.5, probably cannot build enough muscle mass to wear a full suit of medieval armor at all, let alone in two weeks. I do not think we have seen evidence that wizards are stronger than ordinary folk, as opposed to more resilient. The captains are described as wearing only metal shirts, but they practice by swinging metal objects on their hands and feet -- this is odd.
I've worn full-weight chain and plate reconstruction items while running around for a full day, and I'm not physically fit at all - I'd say that a random geeky 12 year old boy would be easily able to wear an armor suit, the main wizard-combat problems being getting winded very, very quickly if running (so they couldn't rush in the same way as Draco's troops did), and slightly slowed down arm movement, which might hinder combat spellcasting. It is not said how long the battles are - if they are less than an hour, then there shouldn't be any serious hindrances; if longer then the boys would probably want to sit down and rest occasionally or use some magic to lighten the load.