Update: Nintendo has since issued an official service announcement/reminder that it is in the best interest of new Wii U owners to allow the system update to complete.
"System updates for Wii U are important and we highly recommend that any Wii U owner does not stop the system update while in progress.
"It may take an hour or more to perform the first system update – the time required depends on the speed of your Internet connection. Powering off the console during the update 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."
If you were lucky enough to score a Wii U yesterday by hitting your friendly local retailers, you'll know that Nintendo actually shipped their first batch of launch day hardware without many of its features installed like the Miiverse, eShop and even backwards compatibilty for Wii software. Fortunately, a firmware update was made available for Wii U owners to download during the initial system setup. The firmware update was not totally mandatory to enjoy many of the Wii U launch titles, but currently it's the only way to be able to explore and make use of these new features which could serve to enhance the console's experience.
This ultimately amounts to around 1 to 1.5GB worth of updates (initially thought to be as high as 5GB) that the Wii U needs to grab off the internet once the console is plugged in, so hopefully prospective owners who want to make use of these new features were savvy enough to keep in consideration the amount of storage space built into their version of the hardware. Wii U Deluxe owners enjoy 32GB of storage space out of the box, but shoppers who opted for the cheaper Basic model will be left with a lot less room after the update!
It didn't help that the crush of new users wanting to get the updates ended putting a serious strain on Nintendo's servers. Users began tweeting and reporting download times of around an hour, and that's before the additional half hour required to actually install the updates. Better find something else to do in the meantime and whatever you do, do NOT pull the plug on the Wii U or otherwise interrupt the system software update download in any way. You'll risk bricking your Wii U as a result, as several unfortunate users have already discovered. Keep an eye out for the Wii U downloading updates in the background too, even if you chose to decline grabbing it for the time being when you first booted up the system.