I can't believe it took me this long. I just realized that videophones, which I normally consider to be a nice-to-have addition to voice conversation (or chat), are an amazing leap forward for deaf person. In fact, it would probably make sense to offer a video-only version of such things, to save the bandwidth of a useless audio channel.
How many other "nice-to-have" features are we ignoring, instead of enabling uses we didn't expect?