The Wii U console requires a big update if you want to get all their Internet features like a web browser and voice chat, none of which are available out of the box. I started the update just before I was going away for the weekend, so when I realized that it was going to take hours to download, I tried to stop it, first by doing things like pressing the B button, which cancels a lot of things, but not that, and then by pressing the power on the gamepad. That turned off the gamepad, but the console stayed on, and while I considered trying to turn the console off, I decided that I just wouldn't worry about it.
That was lucky for me, because as reported in Tech2, an LA Times writer unplugged his Wii U to stop the update and it bricked his console.
Before I turned off the gamepad, I looked on both screens and there was nothing that said, "Warning: Don't turn off console during update." This seems pretty dumb on Nintendo's part, since a lot of people are going to want to delay the update for a more convenient time once they realize how long it will take.
So, keep in mind that updating will take hours, and that once you start it, you can't stop it. And hope you don't have a black out, or you're just plain screwed.