In canon, the diary was a trap: it was blank, but if you wrote in it, it would write back. Invest enough of yourself in it, and Voldemort could use it to steal your body. Also, it gave Arthur Weasley his best line in all seven books:
Haven't I told you kids before? Never trust something that can think if you can't see where it keeps its brain.
...which is relevant enough to MoR's themes that I've been waiting for a subversion to turn up, but no joy so far.
As Voldemorte's horcrux it did that, but I was under the impression that it was originally an actual diary that Riddle wrote in.
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: