No traffic until people actually start using drone delivery systems. It's like with planes, the air seem so big but in fact it's very crowded and you get mid-air collisions all the time (hell, one case was just being discussed on HN earlier today). Scale that up to account for more drones in smaller space.
Apart from that, it still isn't obvious to me how this is better than an electric delivery truck. Cars are much more energy efficient than drones (especially helicopter-like ones), doubly so if you load the car to its capacity. Note that in case of drones, the mass of the package is only a small percentage of the overall mass of the vehicle, so most of the energy used will scale up with number of drone deliveries. I'm pretty sure taken altogether cars will win in terms of energy use.
I think the traffic Amazon would be worried about is human drivers, not other drones. AFAIK, Google's driverless cars have been in 14 documented accidents and all 14 were the fault of human drivers (including one incident of a human driving the Google car). Machines may be better drivers than humans.
Depends on the type of drones you want to use and their range. Helicopter-like drones have very poor range and flight time, so this would limit your range considerably. And if you go to a plane-like drone, or even some kind of hybrid mode (eg. VTOL), suddenly you can't really do an emergency hover when cruising to destination.
But even if they find a way to handle the traffic, I was just saying that there will be traffic and that this poses serious problems.
There are dozens of reasons why an autonomous drone is greater than an autonomous car, use your imagination.