What's fascinating about airplane design for me is not the huge technical complexity, but rather, the way it is designed such that a lot of its subsystems are serviceable by technicians so quickly and reliably, not just in a fully controlled environment like a maintenance hangar, but right on the tarmac, waiting for takeoff.
Designing the airplane to minimize required maintenance and to make maintenance and inspections easier and faster is a huge issue for the engineering department. Also make it very difficult for the mechanics to do things wrongly.
As it was pointed out to me, airplanes sitting on the ground are a black hole sucking up money. Airplanes in the air carrying payload (note the "pay" in payload) are making money. Boeing understands this very well, and is very focused on getting that airplane in the air making money as much as possible.
