First and foremost, the Jehovah's Witness religion denies the Trinity. They also deny the deity of Christ, His physical resurrection, and salvation by grace. Another interesting and not-surprising fact is that they believe that they are the one and true church of Jesus Christ. With all of these facts, a person can conclude that the Jehovah's Witness religion is indeed a cult, not a Christian congregation. 
If you go with the definition of cult from the dictionary, pretty much any religion that are not the major ones are cult, but let's disregard that and go with the common definition of cult.
Make you go at extremes that go against the common sense:
Yes. You may forget that shunning is an 'extreme' case of going against the common sense of the society. For people to cut all communication to their children, parents, siblings, family and friend is extreme after all. They say that it is to convince the person to come back, but it is wrong; it should be for God and the religion itself they should come back, not for the people inside. They say that the person become a threat to their spirituality; here is true, as if someone when out of it, it means that there is a reason and they don't want the people from outside to know. Furthermore, it makes the people having friends and family inside that religion practically unable to leave, as they know they would loose all these people they love. In fact, they actually make people choose between the people they love and their own belief. For common folks, it is simply unimaginable.
Well, nobody would believe to be brainwashed and everyone think to be immune, but let's think about it seriously. When you believe that the true only come from one source and everything is the world of Satan, you kind of cut everything that comes from the outside. If anything against the organisation is bad, you will ignore it as soon as it doesn't agree with what they teach you. If the only things you look at and accept is what come from a small group, if your only friends are the people from that organisation (because people from outside are bad frequentations and you have to limit the contacts) and if you instantly only look into what they give you to make sure that it is the 'true', you are slowly closing a box on your mental faculties and limit yourself at only what they give you and auto restrict yourself. Your are isolated from the 'normal people' because you believe that they are liars, cheaters, alcoholic and all that stuff, and it brings you to extreme behaviors (look for argument one if you don't believe me). What to call if not brainwash?
Adding onto NewLifeChristian their practice of disassociating members is awful. I know someone who was disassociated simply for dating outside of the church and now she cannot speak to her own family. They attempt to keep members ignorant and brainwashed into the "church" . Also, they don't celebrate birthdays, Christmas, or other "pagan" holidays. Jehovah's Witness and Scientology are both "religions" that are only "religions" because of America's very loose definition of a church.
Jehovah's do not make up their rules. They follow what the bible says. They follow what God wants. For example, is right to obey God for one day (Sunday) and then go back to being your normal self on the other days. It's not right. The trinity believes that there are three things making up god. God, Jesus, and the holy spirit but Deuteronomy 6: 4 says that Our god is one god. And why don't we believe that Jesus is god. If Jesus was god and he came to Earth, why did he pray. That makes no sense. And now Birthdays, there were only two birthdays in the bible and in both of them, someone got their head cut off. Christmas was not always the birth of Jesus, it used to be the birthday of the Sun God.