NASA is currently working on creating an "air traffic control" system (ATC) that will make sure the drones that fly below 400 ft do not bump into each other or, say, an office window. But, regardless of NASA's experience in the sphere, the system is still in its nascent stage and it won't start functioning soon. Yep, bad news for Amazon and Google.
The painful thing about delivery drones is that, unlike big airplanes, they fly relatively low and weight less so they can easily crash into a building because of a sudden gust of wind and, on the whole, are 'weather dependent'. Plus in highly-populated areas, there are much more obstacles for a UAV courier, even if we don't count those who would gladly use a drone as a target to practice their shooting skills.
Nevertheless, NASA remains rather optimistic about the commercial use of drones in future and says to the New York Times that it might be possible to start using the new technology in agriculture and pipeline monitoring in about two years. As for the delivery services, NASA says that it might be launched in less populated areas no sooner than in five years, and big cities will have to wait much more.