Opening up my SC2 directory, I found approximately 5GB of files under the "Campaigns" folder, and an extra GB of files under the "Versions" folder.
Having already finished the campaign, I began to wonder if I really need these extra 6GB of game files...
WARNING! removing files from your SC2 directory may mess up your installation, erase saved games, replays and possibly cause severe bodily harm (though the latter is VERY unlikely). Also, note that the Blizzard support staff explicitly stated that you should not delete any game file in this forum post - attempt at your own risk!
That said, I've found that you can delete the two folders under the "Campaigns" directory, as well as all but the latest (highest number) "Base#####" and "Shared####" folders under the "Versions" folder.
I'm told that deleting the old "Base" and "Shared" folders will prevent you from viewing old replays, so you might want to keep them around, since they don't take up that much space.
Withe these files removed, my SC2 installation takes up less than 4GB of space, and I can still play multiplayer games on Battle.net without issues.
