Nintendo Network accounts created on a Wii U are currently tied to that console irrevocably, meaning that if that console is lost or stolen, your account disappears with it, along with any stored information and purchases.
This also means that players can't sign into each others' Wii Us. Each console currently has a limit of 12 accounts which can be created on it and which will be tied to it permanently. Whilst that gives plenty of scope for creating accounts for all friends and family likely to use a single machine, it does mean that second-hand consoles will come with reduced capacity for more, potentially meaning that the limit of 12 has already been reached.
An explanation in the Wii U support literature gives some hope that it may be a situation which changes in the future, but offers no specifics.
Q:Can I Log Into My Nintendo Network Account on a Different Console?
Answer: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.
Currently, the Wii U supports up to 12 individual Nintendo Network ID's per console, which enables each member of the family to have his or her own unique identity on the system.