The big difference with modern multi-rotor UAVs is that unlike a conventional model helicopter or model aircraft they can be flown with ease because they are electronically stabilised by inbuilt electronic gyros and with GPS installed will hold position as soon as the controls are released. Therefore, anybody with the money to purchase one, or the bits and the skill to build one, can then fly it with no training whatsoever.
I own a six rotor UAV equipped for high definition filming. It weighs around 8 kgs. with 12" carbon fibre propellers which are like razors. I also happened to have attended the relevant ground school, passed the written exam, taken the flying test so now hold a BNUC-S and CAA permit for aerial work. To get that I also had to write an operations manual which is akin to that for operating a full size aircraft. All told around £10,000 has been spent but it is used for commercial aerial film work.
I am restricted to line of sight operations which is generally accepted as 400' vertically and 500m horizontally from where I am standing. I am also subject to numerous other (sensible) restrictions regarding how close I can fly to people, buildings, etc., etc.
Unfortunately the widespread availability and cheapness of UAVs means that idiots can have a field day. Someone is going to be seriously hurt or worse and then there will be a major clampdown. I just hope that those of us who take the operation of a UAV seriously will escape whatever draconian legislation ensues.
I am sure it is only a matter of time before a terrorist gets in on the act.