It seems to be a plot hole in MoR (ETA: not in canon - so the zombiehood is important) that no-one who appears on screen seems to have known Quirrel before his appointment as teacher. In particular, no-one ever gets to ask, "why is Quirrel acting like a zombie, he didn't do that when I met him ten years ago". Neither does anyone say, "I know you've been wondering why Quirrel acts like a zombie; he's been like that ever since I met him ten years ago, and here's why."
No-one is holding the idiot ball. Therefore Dumbledore didn't take a complete stranger on as Defense Professor; Quirrel must have had references.
Since the zombie-hood is due to possession/mind control (in canon), we can assume that it implies possession in MoR as well (even if possession isn't true here). Also, Dumbledore remains suspicious of Quirrel. Therefore Dumbledore must have investigated Quirrel's zombie-hood, or gotten a satisfying explanation from Quirrel himself.
Further note: Dumbledore can cast warding spells such as "no-one who wishes to harm the people living here may enter this house" (this was used on Harry's house in canon and was powerful enough to protect Harry from Voldemort and the Death Eaters in the summer vacations). The warding spells on Castle Hogwarts are described as diverse and very powerful, and if they didn't have this function already, Dumbledore would have added it (in canon he presumably was holding the idiot ball and didn't do this). Therefore we know Quirrel isn't planning to harm Harry directly, or at least wasn't planning to do so at the time he entered the castle.
I'm pretty sure the ward on the Dursleys' house was a.) specifically targeted for Harry and b.) specific to Voldemort or those influenced by him, due to c.) it drawing on the lingering force of Harry's mother's sacrifice of her life. A similar ward on Hogwarts in general would probably be impractical. (In canon, Harry dies (or intends to die, anyway) in order to protect the greater body of Hogwarts students, and the protection of that magic extends to them, but not as strongly (Voldemort's magic, instead of just bouncing off, does not bind them fully and w...
[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.