They are not allowed to interact with passengers when not in their uniform during flight.
They can not give kids free seats in the flight.
Flight attendants are not allowed to take tips.
They are not allowed to share a drink with passengers.
They can not serve endless amount of alcohol and can't let you drink that booze you brought onboard.
They can not force any passenger to switch seats.
They can not enter cockpit without permission.
They can not promote their personal business on flight.
Flight attendants can not use or share your personal information.