When Nintendo confirmed the death of friend codes and the establishment of the Xbox Box Live-style Nintendo Network ID, the community rejoiced. However, it seems that for the time being at least the system isn't going to be anywhere near as flexible as Microsoft and Sony's online equivalents.
Nintendo Network IDs are tied to a single machine, which sadly means you can't go over to a friend's house and sign into his Wii U with your own ID. It also means that if your machine breaks or is stolen, you lose your ID and all of the purchases you made with it.
Here's the official rub from Nintendo itself:
Q. Can I Log Into My Nintendo Network Account on a Different Console?
A: No, you cannot. A Nintendo Network Account can only be used on the console where it was created. In the future, you will be able to use your Nintendo Network Account with future Nintendo consoles and other devices, such as PC's.
It's disappointing, but that "in the future" comment adds a glimmer of hope; the Nintendo Network ID still has the potential to become a single, unique sign-in across all of Nintendo's platforms.