After a few months of use, your WoW application folder will get cluttered with large, unnecessary files – remnants of old patches, mostly, which can safely be deleted.
If you’re looking to clear some hard drive space, this thread confirms that you can safely delete any old patch files with “a blue tornado icon or comet icon” except the Background Downloader and Blizzard Updater.
(These are generally files with names like “WoW-2.3.2.7741-to-2.3.3.7799-enUS-patch” or “WoW-2.4.2.8278-to-2.4.3.8606-enUS-downloader”, for instance.)
You can find these files in the base-level World of Warcraft directory.
You can also delete the /Patches subdirectory, although – as a blue poster points out – you might want to delete selectively if the downloader is pushing content in advance of an approaching patch.