Update: Discussion has moved on to a new thread.
The hiatus is over with today's publication of chapter 73, and the previous thread is approaching the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it.
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. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
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:
You do not need to rot13 anything about HP:MoR or the original Harry Potter series unless you are posting insider information from Eliezer Yudkowsky which is not supposed to be publicly available (which includes public statements by Eliezer that have been retracted).
If there is evidence for X in MOR and/or canon then it's fine to post about X without rot13, even if you also have heard privately from Eliezer that X is true. But you should not post that "Eliezer said X is true" unless you use rot13.
Cannon!Snape is definitely badass, but I always figured that given Hogwarts' eminent reputation and the size of their faculty, their standards for anyone holding a professorship are probably extremely high, barring positions people don't want to fill (Defense Against the Dark Arts and possibly Care of Magical Creatures) and pity jobs like Trelawney.
In a setting where the academic and practical are so closely intertwined, I imagine holding professorship at Hogwarts as being something like being a member of the National Academy of Sciences and Green Beret combined, probably topped off with an Eagle Scout.
It is a damn high bar and I think you mentioned the big players. Dumbledore -> McGonagall is easy. Then it is Snape vs Flittwick for No.3. It wouldn't seem unreasonable to place Snape higher than the gnome illusionist given that Snape is the one with dark secrets that no doubt include all sorts of badass juice. The problem with the '3' figure in MoR is that Quirrel comes in as a wild card and you would be reckless place him anywhere below second. And as you say, Snape > McGonagall doesn't seem right!