Consider this scenario-- which remains, as far as I know, science fiction: a United States president is assassinated by a tiny micro-drone.
Now what happens?
Well, killing a US president by itself does very little to the United States-- the chain-of-succession is well-defined, and really they're all 2-term short-timers, and arguably their power and influence is often exaggerated.
The actual trouble is the panic it would cause. In recent years, the United States has not shown itself capable of measured response to threats. If we have nothing to fear but fear itself, then you should be very afraid.
I'm skipping the details of how the public actually learns about it-- you can imagine a cover-story like "heart attack" being preferred to admitting what actually happened-- what I'm interested in is what happens when people realize there's a new, dangerous technology out there.
Guarding someone like a US President against this particular threat might turn out to require only relatively simple measures such as fine steel mesh faraday cages (at least in the early days of the technology)... but it's unlikely we would stop with those simple measures.
hackers and hobbyists
At present, I know of three or four "hacker spaces" in the Bay Area I have access to (or could get access to), with facilities like 3-D printers and CNC machining tools. I expect that there are government agencies already quietly keeping an eye on them, but it's difficult to see how they could survive at all in a world where a micro-controller looks like an assassination tool.
There's a park in Sunnyvale that often has people out flying radio-controlled model airplanes and helicopters. That's a hobby that I fear is not long for this world: it will be suppressed instantly after an assassination scenario like the one I've sketched out here... and in fact, I expect it would happen even if someone used a model plane to spy on government agents doing something sleazy and illegal. (In which case, someone like myself might have played into their hands by just talking about hypothetical assassination scenarios that they can use as an excuse...).
There's already quite a few people who have trouble distinguishing between a Hacker and a terrorist, e.g Hackers Will Replace Terrorists as Top Threat, Says FBI.
once accepted, never banned
There is perhaps one ray of hope for the "private drone": I think you can see a pattern where dangerous technologies are never suppressed if people (particularly, rich people) first begin using them for "conventional" purposes. There are entire industries surrounding cars, cellphones, and nitrogen fertilizers so that even if they do get used by "terrorists" on occasion they aren't going away. If the likes of Amazon or Google can get a drone-delivery service established before a drone assassination occurs, then there's some hope for the model plane.
Note that it's best if upper-class people like the technology. I have a theory that cars would be illegal to this day if they started out used by dirt-ball drag racers, rather than as a toy for the well-heeled. Think about the problems skate-boarders have.
(And I suppose guns might be taken as a good example of a technology that would probably be prohibited if invented today, but are already an accepted part of the culture... but I'd rather not talk about gun control here today. You all heard that, right? This is not a gun control thread, okay? Oh well.)
hackers as engines of progress
Obviously, we can live in a world without model planes, but there's a serious downside toward banning/heavily-regulating such "toys": there's a serious impact on technical progress.
A generation of new hacker/engineers may be suppressed, or perhaps run off to other countries in search of the freedom to tinker with such things... presuming they can find a country with that freedom.
Note: For some time after the Dimitry Sklyarov incident people were afraid to hold certain technical conferences here in "the land of the free", because courtesy of the Digital Millennium Copyright act, talking about math can be regarded as a criminal offense.
pundits at play
Imagine the punditocracy weighing in on this (if you want to feel really tired): how can it be that our commitment to torture, assassination and wholesale electronic surveillance didn't prevent this tragedy? The winning argument would almost certainly be that we weren't doing enough of it.
Just as an aside: wouldn't it be cool if we could fix our culture before we go off into another cycle like that? Wouldn't it be nice if we had some damping force in play, some sort of social institutions that encouraged people to calm down and look a little further than the present crisis?
real fears
That covers the inevitable overreaction-- but it's not like a breakthrough in assassination technology wouldn't be a real problem.
Once a micro-drone is used in an assassination, that's a very powerful advertisement for the technology. Imagine all the different factions out there in the world that might have a use (or think that they do) for anonymous, untracable assassination. Consider the diverse range of possible motivations: military, political, racial, religious...
It might very well not be a widespread threat of any sort at the outset, when it's a new and experimental technology, but in spite of any attempts at suppressing or controlling it, it's bound to become cheaper and more ubiquitous.
We can look forward to decades of frantic research into anti-drone-drones, with every public place buzzing with artificial, weaponized insects...
The defense side might even win that arms race... for a while.
predecessors: science fiction
Some predecessors to these ideas can be found in the world of Science Fiction, of course. John Shirley's Eclipse has a throwaway detail, a flying robot carrying with a spy camera, disguised as a bird. There was a short-story that appeared in Analog in the 70s with assassination drones like artificial insects with a sense of smell that could home-in on a biochemical signature.
state of the research
After writing the above I did a web-search to see what the state of the art is on drone miniaturization. The phrase "micro-drone" (which I thought I made up) has been in use for a number of years, now.
military
The US Air Force has been working on such things since 2008, according to David Hambling at Wired.
And from the Daily Mail in the UK, a story from 2013 (and a fine example of modern headline writing): Death from a swarm of tiny drones: U.S. Air Force releases terrifying video of tiny flybots that can can hover, stalk and even kill targets
hobbyists
And there's already some hobby projects like this, see this review of the "Micro Drone 2.0", a palm-sized quadrocopter, that at present can fly for around 8 minutes, maybe 5 minutes if using the camera: The Micro Drone 2.0 With An Aerial Camera: Fly And Spy In The Sky. And you can see it in action in this video.
An indiegogo campaign raised 2.5 million dollars to develop Micro Drone 3.0: Micro Drone 3.0: Flight in the Palm of Your Hand