"Sense and avoid" is the current holy grail of UAV research.

Restricted zones have some interesting effects. DJI, a Chinese multirotor manufacturer, recently introduced 'no-fly zones' around airports for their ultra-consumer-level uav, the phantom 2. An unfortunate side effect, in the UK at least, is you are now unable to fly your phantom 2 in areas where it is perfectly legal to do so. I believe this limitation also effects where you can place waypoints on their more advanced autopilots.

They also prevent you flying over or near Tienanmen square...