So tons of consoles or games or services have a picker where you can easily pick an account to use without entering a login and password.
Well, so I made a lazy edit of the Wii U picker screen:
This implies that there will be KNIDs (there won’t, Jake confirmed it), and a nickname system (which was mentioned).
Confirmed in this post:
In PW, there would be rows stating the PW username and status (Logged in/Not Logged In). If an account is picked that is NOT logged in, you will be asked for the password (not login as the login is already provided).