[Update: and now there's a fifth discussion thread, which you should probably use in preference to this one. Later update: and a sixth -- in the discussion section, which is where these threads are living for now on. Also: tag for HP threads in the main section, and tag for HP threads in the discussion section.]
The third discussion thread is above 500 comments now, just like the others, so it's time for a new one. Predecessors: one, two, three. For anyone who's been on Mars and doesn't know what this is about: it's Eliezer's remarkable Harry Potter fanfic.
Spoiler warning and helpful suggestion (copied from those in the earlier threads):
Spoiler Warning: this thread contains unrot13'd spoilers for Harry Potter and the Methods of Rationality up to the current chapter and for the original Harry Potter series. Please continue to use rot13 for spoilers to other works of fiction, or if you have insider knowledge of future chapters of Harry Potter and the Methods of Rationality.
A suggestion: mention at the top of your comment which chapter you're commenting on, or what chapter you're up to, so that people can understand the context of your comment even after more chapters have been posted. This can also help people avoid reading spoilers for a new chapter before they realize that there is a new chapter.
Eliezer planted lots of clues about many facts that are never explicitly revealed, in such a way that noticing correct hypotheses is sufficient to confirm them upon observing enough of those little clues. Now, for some facts, it could be difficult to even locate them, but Quirrell=Voldemort seems to be a good hypothesis to entertain, even if it's not apparently confirmed from any single passage, and it does get lots of evidence if you know to look for it.
How much of that is hindsight bias? Clues that show a specific hypothesis if you've located the hypothesis aren't necessarily that helpful. For me at least, even knowing the that Q=V, and seeing the clues, they don't intrinsically point to that. Most of them can be explained simply by the idea that Quirrell is subtle, evil, and likes corrupting people.
The biggest clue is the material about the Horcrux and if one hasn't read the books that likely goes completely out the window. (In fact, if I were Eliezer, I'd have Harry find out about Horcruxes pretty soon to help the less knowledgable readers.)