All consoles offer an automatic shutoff system. It saves energy. Prevents overheating. Makes sure you don't accidentally leave it on overnight. It's designed to make your gaming life easier by removing the tenuous task of actually shutting it down yourself. That's great and all, but it can also be a colossal pain in the ass. Especially with the new Wii U from Nintendo.
Most consoles have their automatic shutoff disabled out of the box. You have to enable it to the time setting of your choosing. The Wii U does not. It comes preset with a 1 hour shutoff and requires you to disable or extend that time in the console settings. It may not seem like an issue, but it can be if you aren't vigilant at the start.
The Wii U considers a paused game as "idle" and will shut down at the end of that one hour. If you pause the game because you haven't reached a save point, and you head to the store for a couple hours, you'll come home to find out nothing you did will have counted and will have lost a lot of gaming time. If you're planning to spend a lot of serious time with the Wii, make sure you disable that shutoff.