The only way I could imagine this working in even moderately densely populated areas is with some sort of landing pad. Maybe it's as simple as laying out a ~3' square cardboard pad with a QR code or some other fiducials, or maybe as complex as an active beacon or GPS enabled pad.

Avoiding powerlines seems like a fairly tough problem as well. Are drone-sized object sensors (ultrasonic? LIDAR?) accurate enough to see thin power lines?