I feel it's acceptable to take off ones shoes during a flight -- especially long haul flights. But that said, this ain't nam so there's gotta be some rules:

1) You may take off your shoes but you may not take off your socks. If you are not wearing any socks your shoes/sandals must stay on at all times unless you immediately and expeditiously put on travel socks that were provided in the amenity pack.

2) While your shoes are off your feet must remain on the floor at all times. Do not put them on the bulkhead, the seat back, the arm rest, and definitely by no means on tray table or the IFE.

3) If a visit to the galley or lavatory is in order you best be prepared to put your shoes back on prior to leaving your seat.

4) Shoes must remain on during takeoff and landing. If there is an emergency incident during takeoff or landing, I will not play the game where you delay everyone else from an expeditious evacuation just because you're shoeless.