Just checked out some iWatch design predictions
Like the talk - here are some thoughts.
Acceleration
We have been doing research into physical interfaces. One important possible interface element is that you get orientation/movement which you can use for a lot of interface activity. Myo ( MYO - The Gesture Control Armband https://getmyo.com/?goback=%2Egde_753787_member_217567055) incorporates nerve sensing technology to give finger/ hand shape position. With this your interface becomes more like Harry Potter hand waving. Imagine pointing to the TV palm out like a policeman to stop it. Finger flick to carry on.
Accelerometers means you could have a simple fitness app - if you could measure pulse then you could save about 80,000 US lives a year by detecting early signs of cardiac arrest. Sleep apps for phone are very popular this might be another early big market for iWatch apps.
Our research work
One of our experiments was helping to people learn to practice violin ( getting arm/body position is important). So the use of a vibrotactile element (vibrator) is very important ( helps get your attention sent info back). You could program these things to help with learning drumming or ballet or other physical skills. Some people done good work with relearning to walk after strokes - a bit specialised but shows the potential for novel interaction with iWatch. So the iWatch could be the Wii Fit of the mobile gaming world.As far as the you identifier - you could make it use body electricity to modulate a unique signal to each device you touch. Basically if your wearing your watch your system knows it's you the moment you touch it. No long in screens just touch. Same goes for the 3 card reading locked doors you have to pass in order to just go to the loo. Could be replace by pressing the flat plat as you push the door.