It's hard to believe, but the Wii U is the first Nintendo system to allow for background downloads. It even does one better than the Xbox 360 and PS3, as it's not just game downloads that can be performed in the background—it's mandatory updates, too.
So when you boot up a game that you haven't played in a while and it tells you you need to download an update, you can still play the game while it's happening. Then the next time you start up the game it'll ask you to install it. It's so convenient that we're shocked it hasn't been done on another console before.