So apprently the patch requires 6.5GB of space to install (I had 4.7 free). What it's doing with that space, I have no clue (6.5GB is a lot, it's more than half what WoW takes up at the moment).
So it was goodbye to the Diablo 2 (1.88GB) and Starcraft (1.2GB) directories.
No idea what I'll be deleting for WOTLK, Windows maybe.
*This post is dedicated to Out Of Mana.