I could have sworn that somewhere I saw a comment expressing surprise or disappointment at Snape being taken down so easily -- but I now can't locate the comment.
In case it was someone from here: It seems clear to me that it was Quirrel who both chose to reveal Snape's location, and of course he'd have also contributed to the volley of spells that quickly took down Snape's shield as well. It seems that Quirrel really didn't want Snape getting the situation back under control - and his presence was what Snape didn't anticipate.
You can't raise and strengthen very much in the way of advanced shielding and be invisible at the same time, and massed fire from forty-four fifth-year through seventh-year wizards is nothing to sneeze at. Snape would have noticed Quirrell taking him out, but it wasn't necessary for Quirrell to do so.
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: