I have serious doubts about the practicality of aerial drones, at least for deliveries to individual consumers. What happens when a drone shows up at your door and the family dog attacks it? There are many other problems which other posters have mentioned.
I would really like to see autonomous, road-based drones developed. A road-based drone could be much smaller, lighter, and cheaper (in mass production) than a car, since it wouldn't need to carry people. Road drones would need the same sorts of sensors as self-driving cars, so if road drones were mass produced, self-driving cars could reap the benefit of cheaper sensors. Road drones also would need software that would be very similar to that required by a self-driving car. But a road drone, being smaller, lighter, and cheaper, would usually cause less damage if it were involved in an accident. So it might serve as a testbed for new software, before it is deployed in self-driving cars.
The trick would be to come up with a form factor that could share the roads with existing traffic. I was thinking of something the size of a bicycle, a Segway, or a very small car. But it just occurred to me that there is a lot of space underneath most cars. Maybe a road drone could just position itself under a car and stay there as long as the car is going its way. Making a transition to another car if a drone's car turns in the wrong direction would be tricky, and perhaps not even possible if all the surrounding cars have their own drones. Ideally you would want the humans driving their cars to be able to completely ignore the drones, as the drones would be smart enough and fast enough to keep themselves from being squished. Of course it would be easier if there were a substantial number of self-driving cars on the road, and the drones and cars communicated and coordinated.
I would love to work on developing something like that.