You’ve probably seen in any number of games, or even on your PC not to turn it off while updating or saving. Nintendo is reminding users however due to the lengthy initial update for the Wii U. They “highly recommend that any Wii U owner does not stop the system update while in progress.”
Updating may take an hour or more the first time depending on your internet speed. Powering off during this may damage your system. The update enables numerous features including Miiverse, Wii U Chat, the eShop, and the ability to download video-on-demand applications.