The smart bicyclist communicates where s/he is going because the stakes are so high. Cars are hard and bicyclists are squishy. There are a wide variety of turn signals available, but what makes Shield notable is that its completely hands-free operation. The rider can continue with both hands on the bars. Just turning the head (or, rather, the helmet) does the work. The primary challenge, I think, is having the option to look around without accidentally signaling a turn.
This is one of many bicycle design innovations of the past year rounded up by Rain Noe of Core77. Others include heated handlebar inserts, a bike with a built-in workbench, and a bike with sawblades for wheels.