Just as one wants to minimize obtrusiveness of wearable computers, so is it desirable to minimize the obtrusiveness of methods of communicating with it, such as sign language. To this end a good sign language should be usable head-up as with touchtyping.
It would also be very useful if one could type even when very much engaged in some activity, e.g. while jogging, or lying on one's back under an automobile.
And it goes without saying that the faster one can type with any given system the more useful that system.