A current theory is that it contains the Philosophers stone, its certainly more important than just a rock.
It's certainly not the Philosopher's Stone. The only reason the stone isn't at the Ministry (or Gringotts) is that Hogwarts provides the absolutely best protection:
"I do not own it, that thing which Voldemort desires. It belongs to another, and is held here by his consent! I asked if it could be kept in the Department of Mysteries. But he would not permit that—he said it must be within the wards of Hogwarts, in the place of the Founders' protection—" Dumbledore passed his hand across his forehead. "No," the old wizard said in a quieter voice. "I cannot pass this blame to him. He is right. There is too much power in that thing, too much that men desire. I agreed that the trap should be laid behind the wards of Hogwarts, in the place of my own power" (Ch 79).
I can't see Dumbledore going and giving it to Harry to wear on his finger.
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: