Without a doubt I think it is the door to door work.
It is the perfect example of the influence of consistency. Humans have a natural need to be consistent. Their actions need to be consistent with thoughts and emotions. That is the basis for cognitive dissonance theory.
Also it has been shown that if a person performs and act that is difficult (knocking on strangers doors to recruit people is VERY difficult!!!), they will have a stronger need for their future actions to be consistent with that act. If an act is somehow public (door to door is seen by fellow jws and people being preached to), then the need to be consistent is further reinforced by the outside social pressure that comes from needing to be seen as consistent by others.
You could probably add to that the way it divides the world in the jw mind. They are the right ones. All others are potential recruits or enemies. The only legitimate way to interact with the outside world for the jw is the preaching work.
Prostletyzing = Indoctrination
The jw's may not be the most controlling cult out there, but they are probably one of the hardest from which to free a person's mind.
CYP