One of the biggest problems with computers is that you can't dance and type at the same time.
In a world where people spend ever-longer periods of time in front of computers, the fact that interacting with the computer general requires a near-motionless seated posture has major implications for the health and fitness of society as a whole.
Perhaps my best unrealized invention is the Aerobic Keyboard, which would use cameras to recognize large gestures made using your arms and legs, and interpret them as characters typed. This solves the typing and dancing problem by turning dancing into typing. However, gestures vigorous enough to give you a decent workout might be problematic in small cubicles.
I was talking the other night with a guy in Second Life who solves the Typing and Dancing Problem in a different way. He has a copy of Dragon Naturally Speaking, the speech recognition software. When he visits a dance club in SL, he stands in front of a big-screen monitor with a wireless headset microphone and a wireless mouse, cranks up the music, and dances along with his avatar. When he wants to say something, he lets Dragon turn speech into type. He assures me that Dragon is smart enough to decipher what he's saying even over the music.
(In most places in Second Life that play music—and there are a lot of them—voice chat is either turned off or discouraged, as I mentioned in my previous post.)
I've never used speech recognition software, but even if it is as accurate as claimed, I have some doubts about the wisdom of using it when you're not alone in a room. Won't people constantly be asking, “Who ya talkin' to?” But I'm intrigued enough by the concept that I ordered myself a copy of Dragon, which is supposed to magicking its way to me via UPS at this very moment.