When they can do equivalent things to opaque overlay--
Realtime text translation, with proper angle and shading of the overlay. EG-- can look at the street sign in arabic, and it looks like it is in english, because it totally replaces the content of the sign with the translation, and keeps view angle in registration.
Another potential killer app is in medicine. There exist very sensitive cameras that can actually detect the pulse of a human, by measuring the color intensity variation of the skin, then enhancing it with an algorithm.
http://www.wired.co.uk/article/mit-algorithmAdding this to augmented reality would give doctors "vampire vision", allowing them to see if extremities are getting bloodflow, if the person has a pulse without having to measure one manually, etc. This is useful for determining if a patient has a blood clot, or other obstruction to blood flow, which would be very useful as a quick diagnostic tool. (Currently, a doctor has to use a doppler sonar to look for a thrombus (clot), and has to rely on other indicators that one is present, such as localized cyanosis (purple/blueness). Doctors could detect smaller clots more quickly with such a tool.)
So YES, I would say there are already some potential killer apps out there. Just not for ordinary people, aside from automatic sign translations, which several companies are already working on.