However, if weirdness alone is the determining characteristic of a cult, the Catholic Church could be considered a cult
Steve Hassan, cult exit counselor and mind control expert said the Catholics were once a cult. They could become one again if the right nut job gets into the right position of power.
And to answer your question, Yes, the JWs are a cult. If anyone tells you that they, and they alone, have 'the Truth' then you're dealing with a cult. As a JW, you are not to have much, if anything, to do with what is deemed non-JW (aka 'worldly') by the Watchtower which designates them as isolationist.
Once baptized, you are not allowed to question the Governing Body or anyone in a position of authority. You are 'shepherded' if you do and if you continue, you are kicked out. Once kicked out, you are shunned. So they use fear of being cut off from everyone you know and love if you do not conform. They have legalistic views and rules on matters that they should not. My sex life with my partner is none of their business, as are my choices for medical treatment, especially if it is something that can save my life. They oppress a whole segment of their membership based on gender.
I really think agonus states it succinctly and accurately here:
Wacky beliefs don't in and of itself make you a cult. Intellectual dishonesty, manipulation, coercion, etc. carried out to inhuman lengths to ensure said wacky beliefs are followed at all times without exception - now that, I think, qualifies.
