FEAR!
Fear is the biggest and most powerful tool of control that the WTBTS holds over its members.
If someone even dared to mumble aloud against the Organization they risk Public Reproof and Disfellowshiping. Since the reason for disfellowshiping and reproof is cleverly alway kept secret, that person is then gossiped about and slandered in generous proportions. Before you know it, they become, in the minds of their fellow slaves, a serial fornicator, adulterer, (since JWs prefer and love sexual misdeeds to gossip about) or any other vile JW 'criminal'. The gossip mill keeps JWs under fear and control. Bloody Clever!!!
Think how faders are considered by the average JW. They are avoided and gossiped about. 'Surely they left God's pure organization because they could not live up to Jehovah's Standards and wanted to be godless like the rest of the world!?'
JWs police themselves! Any 'mumbler' is quickly rumbled!!