My rule for user interfaces: A GUI is as good as it is easy to help someone use it verbally. The best interfaces have unique handles, names and minimal hidden features. The interface doesn't necessarily need to have tons of labels, but everything on a screen should be identifiable by name.

If you can guide someone through an app while sitting next to them, or over the phone, it's a good GUI. If you have to say, "The grey thing on the left. No, the other one. In the box. It's a rectangle with a sort of an arrow thing," then that's a sign the UI sucks.