The trick is to ignore personality. Never mind how calm or mean someone seems. Just ask: which characters show actions and knowledge that are distinctive to Voldemort?
In canon, Quirrell could not touch Harry because he was Voldemort. In the fic, Harry and Quirrell also cannot touch.
In canon, a Horcruxed object becomes especially long-lived and durable, and the maker of the Horcrux tries to hide it or get it out of others' reach. In the fic, Quirrell tells Harry he enchanted the Voyager 2 space probe to make it super-durable, and talks to Harry about where to lose objects so they'd never be found.
Voldemort knew how he behaved with Bellatrix Black, and is almost the only person with strong reason to rescue her. Quirrell knows and tells Harry how to behave with Bellatrix Black, and persuades Harry to rescue her.
Dumbledore identifies the Bellatrix rescue as bearing the style of Voldemort. Quirrell designed the Bellatrix rescue.
Dumbledore identifies the Hermione frame as having been done by Voldemort. Quirrell was the one who found the bodies, and is the only wizard in Hogwarts we know to be a post-Voldemort newcomer to Dumbledore's acquaintance.
"Quirrell" admits to the interrogating Auror that he is an imposter.
It's easy to miss because we don't think that a personality like Voldemort could turn so calm and un-sadistic as Quirrell. In fact that's the whole background to my post above -- that it's weird how much MoR!Quirrell's leadership style differs from MoR!Voldemort's. How could be so dumb in the past and so clearheaded now?
But once you allow for the possibility of a personality change, or an incredible Occlumens-style personality disguise, Quirrell is the overwhelming candidate. Just watch knowledge and action, not attitude.
past voldemort seeming dumb should also clearly be at least partially the effect of the winner's narrative. (is there some name for this?)
The new thread, discussion 13, is here.
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. With three chapters recently the previous thread has very quickly reached 1000 comments. The latest chapter as of 25th March 2012 is Ch 80.
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, eleven.
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: