There is a long article on this at http://www.jwfacts.com/index_files/mindcontrol.htm
The word cult means different things to different people, so often it is best to avoid that term and instead discuss techniques of mind control. One of the first and best books on this was by Lifton, and he has 8 identifiers of a cult. The Watchtower Society fits all eight points.
1. Environment Control. Limitation of many/all forms of communication with those outside the group. Books, magazines, letters and visits with friends and family are taboo. "Come out and be separate!"
> “False religious propaganda from any source should be avoided like poison! Really, since our Lord has used “the faithful and discreet slave” to convey to us “sayings of everlasting life,” why should we ever want to look anywhere else?” Watchtower 1987 November 1 p.20
2. Mystical Manipulation. The potential convert to the group becomes convinced of the higher purpose and special calling of the group through a profound encounter / experience, for example, through an alleged miracle or prophetic word of those in the group.
> Russell – “Then I knew why the Lord had led me to it so slowly and cautiously. I needed a special preparation of heart for the full appreciation of all it contained, and I was all the more assured that it was not of my own wisdom; for if of my own why would it not have come at once?” Zion’s Watch Tower 1906 July 15 p.234
> Rutherford - “Enlightenment proceeds from Jehovah... and is given to the faithful anointed.... the remnant are instructed by the angels of the Lord. The remnant do not hear audible sounds, because such is not necessary. Jehovah has provided his own good way to convey thoughts to the minds of his anointed ones.” Preparation p.64
> Governing Body - “…Christ used his authority by actively leading his congregation on earth. He did this by means of the holy spirit, the angels, and the governing body made up of the 12 apostles and the elders of the Jerusalem congregation.” Watchtower 1987 August 1 p.15
3. Demand for Purity. An explicit goal of the group is to bring about some kind of change, whether it be on a global, social, or personal level. "Perfection is possible if one stays with the group and is committed."
> “The resulting peaceableness of Jehovah’s people makes them a refreshing oasis in a violent world. ” Watchtower 2002 July 1 p.17
4. Cult of Confession. The unhealthy practice of self disclosure to members in the group. Often in the context of a public gathering in the group, admitting past sins and imperfections, even doubts about the group and critical thoughts about the integrity of the leaders.
> “So, if doubts, complaints, or apostasy threaten to contaminate you spiritually, cut them away quickly! (Compare Matthew 5:29, 30.) Get help from the congregation elders.” Watchtower 1989 October 1 p.18
> “It is certainly not easy to confess to others deeds that one feels ashamed of and to seek forgiveness. It takes inner strength.” Watchtower 2001 June 1 p.31
> “If he does not do this within a reasonable period of time, concern for the cleanness of the congregation should move you to report the matter to the elders” Watchtower 1989 October 15 pp.14-15
5. Sacred Science. The group's perspective is absolutely true and completely adequate to explain EVERYTHING. The doctrine is not subject to amendments or question. ABSOLUTE conformity to the doctrine is required.
> “First, since “oneness” is to be observed, a mature Christian must be in unity and full harmony with fellow believers as far as faith and knowledge are concerned. He does not advocate or insist on personal opinions or harbor private ideas when it comes to Bible understanding.” Watchtower 2001 August 1 p.14
6. Loaded Language. A new vocabulary emerges within the context of the group. Group members "think" within the very abstract and narrow parameters of the group's doctrine. The terminology sufficiently stops members from thinking critically by reinforcing a "black and white" mentality. Loaded terms and clichés prejudice thinking.
> ‘the truth’, ‘new system’, ‘worldly people’, ‘disfellowship’ ‘Jehovah’s Organization’, ‘RV’s’, ‘door to door’ and ‘theocratic’
7. Doctrine over Person. Pre-group experience and group experience are narrowly and decisively interpreted through the absolute doctrine, even when experience contradicts the doctrine.
> “The world is filled with unhappiness, and people generally have a gloomy outlook on the future. However, we have a bright outlook, knowing that one day all sadness will be a thing of the past.” Kingdom Ministry Feb 2002 p.1
8. Dispensing of Existence. Salvation is possible only in the group. Those who leave the group are doomed.
> “To turn away from Jehovah and his organization, to spurn the direction of “the faithful and discreet slave,” and to rely simply on personal Bible reading and interpretation is to become like a solitary tree in a parched land. ” Watchtower 1985 June 1 p.20
> “I suffered much pain and heartache without Jehovah’s guidance,” says one young woman who for a time left God’s organization. “I tried to fit in with the world, but because I was not truly like others, they rejected me. I felt like a lost child who needed a father to guide me. That is when I realized that I needed Jehovah. I never wanted to live independent of him again.” Watchtower 1998 October 1 pp.10-11
> “With apostates earth wide being destroyed, what reason can one have for confidence of life in paradise to follow?” Kingdom Ministry September 1973 p.6