Re: Some interesting possibilities here...
1) Hardly, lift to weight ratios on the drones you'd use for this are pretty good.
2) On the ground, typically. Balconies, gardens, etc. You will probably be able to indicate where you want to meet the drone yourself.
3) Why would it knock when it can call your cell?
4) If you're ordering something by drone, the whole point was delivery in 30 minutes or less. You won't be out, because you're ordering this and you know it'll be with you in 30 minutes.
5) I'm not sure about this one, but probably millions of people, if the location of the Amazon distribution center nearest to me in Netherlands is any indication.
6) Blocks of flats tend have to have balconies, which many drones would be able to land packages on. They also tend to have open spaces nearby you could use instead.
7) Trees and birds are a total non-issue, they are highly visible on LIDAR, which is the most likely choice for collision avoidance. Powerlines and phone lines may be an issue if you live an area where these are common and for some reason not mapped in advance. It's not like powerlines move around a lot. Clotheslines would be a bigger threat, those aren't as easy to map and if there's nothing on them, their LIDAR return tends to be pretty small as well.