ATC, see and be seen
The voice link is a big issue. There was a good piece in New Scientist last year. It said that ATC controllers were not happy with UAV pilots using the phone. ATC were insisting on talking to UAVs using their normal VHF radio systems. This implies that the UAV should carry a remotely controllable VHF radio and a voice link from this radio to its pilot.
New Scientist pointed out another problem. What with UAV control and telemetry channels, surveillance and pilot-eye TV downlinks plus the pilot's two-way voice link, a UAV will be a bandwidth hog. However, the developers have forgotten to book any frequency allocation for UAV operations. Maybe this will mean we don't see them any time soon.
As to see and be seen: either ADSB or FLARM would do the trick for UAVs as well as helping real pilots avoid the robots.
ADSB has been foot dragging for years and is apparently both cash and power hungry, but if/when it gets rolled out it would have the advantage of working with current transponders as well as TCAS and PCAS systems.
FLARM is already in wide use in Europe and Australia and does the same job as ADSB. Its cheap, small and needs little power, but it uses a different frequency to transponders and is relatively short range. Its suitable for gliders, GA aircraft and helicopters but not for anyhing much faster.