I'm somewhat surprised there wasn't a mention of how valuable this would be for deaf users. For instance, they might be able to use it to synthesize voice through hand commands to talk more easily with non-deaf people (those who don't know sign-language).
If the device is made viable for gaming and mobile phone usage, there is the potential for many other users to benefit as well with other applications of the device.
In order to be useful, hand gestures are not something that should be tied to a specific input device as they have been up until now. The goal should be for a vocabulary of gestures to work consistently across all devices. e.g. You should be able to adjust volume by turning an imaginary knob whether you're working with a tablet and using this wrist-sensor, sitting at a desk with a web-cam, or sitting on your couch in front of a Kinect sensor.
Microsoft is positioning itself to be the author of leading OS's running in all of these environments, so they should build in gesture support that uses any form of gesture capture available and possibly even pools all devices available in a given instant to improve fidelity. e.g. These wrist devices are unlikely to be perfect, nor are Kinect devices, but both of them working together should be able to improve overall fidelity.
If Microsoft does this well enough, it may one day seem strange to think of a computer that cannot "see" you. Perhaps that's a bit creepy...
I could see it being useful for guitar players who need to be able to feel the strings, but at the same time I doubt the sensor would be able to see your fingers. At least when I play, my wrist is directly behind the fretboard. This sensor wouldn't see my fingers at all. It could see my pick hand though.
hmm why not measure either tendon pressure or electrical impulses, of course it would need to be calibrated but it could be something the size of a watch instead of this.
If the device is made viable for gaming and mobile phone usage, there is the potential for many other users to benefit as well with other applications of the device.