It's the TV remote.
The Wii U GamePad features built-in universal remote functionality, allowing players to use the touchscreen to turn their TVs on and off, change channels, switch between various inputs, and control volume. It works with cable boxes as well. A "TV" button on the GamePad brings up the TV controls at any time.
Okay, maybe that doesn't seem like a big deal. But it's an improvement in basic usability that accomplishes what every game company wants to do with a system: remove every possible barrier to its use, no matter how insignificant. If I want to play Nintendo Land, the only thing I have to pick up is the WiiPad: I can turn the TV on, switch over to the right HDMI input, turn the system on, and play the game with a single device.
People may not even realize how accustomed they become to this kind of convenience, until they go back to something that doesn't offer it. In the next console generation, we'll look at a controller that can't turn on a TV like we look at a controller that doesn't have a console power button now. I don't think it's a feature that will sell Wii U systems, necessarily, but even this tiny tweak is going to change how console controllers function from now on.