I think a significant portion of my brain (Similar in size to the knowledge from my Bachelor's degree) was dedicated to remembering every quest and map up to WotLK. It's surprising how much I still remember from 7 years ago.
Funny enough, that might also reveal something about how human brains store information: locations on a map sounds suspiciously compatible with the method of loci (aka "memory palace", thanks to a more recent incarnation of Sherlock Holmes)
IIRC there is recent research confirming it is at least partially stored in the same are responsible for muscle memory, which supposedly explains quite a few things.