WoW will work fine in just a folder, I have copied the program files for it several times, and just plopped it in a folder to run it. Did not have to mess with any registry settings. But if others could not, maybe it won't work all the time. I have 4 computers running off one full setup, by just doing a folder copy. When I get a huge patch, I download it on one PC, then copy the files over to the others from the network.
I don't think you can do this for Steam, but check their options for a backup utility or somethign similar.