In GDM <=3.0.X, the highlight would be on the first user when loaded. Although this maybe fairly useless for multiuser systems, on my single user system, all I would have had to do is hit space then type in my password. Currently, I have to first select the user (arrows and whatnot or mouse), which is a bit annoying.

I'm sure there are a fair number of single user systems to make gdm either to have the old functionality or to show the password prompt immediately if only one user is found. In the latter scenario, the "not listed?" could be listed below the password field, accessible via the arrow keys.