I have never been able to come up with an objective definition of "cult" other than a religion that a given person doesn't like.
Here is an excellent cult checklist.
A cult tends to isolate its members from the rest of the world.
Well, like so many other things, there is more than one definition of a cult. There is the mind-control definition given in an earlier link (e.g., Jim Jones), but this is not the meaning of cult an Evangelical Christian is applying to Mormonism. In this definition a cult is seen as a religious group that does not adhere to orthodox Christianity in the following ways:
1. Jesus Christ as a person of the Triune God. Fully God and fully man. Born of a virgin, sinless, dying on the cross for the sins of the world, resurrected after 3 days, ascended into Heaven, and coming again.
2. The Holy Spirit as a person of the Triune God. Convicting the world of sin and pointing them to salvation through Jesus Christ.
3. People are saved by grace through their faith in the finished work of Jesus Christ.
The group may say that they hold these beliefs, but a closer discussion reveals that they are not in agreement with orthodox Chrisitianity. Note that with all the disagreement between Evangelical Christians and Roman Catholics, an Evangelical Christian does not disagree with a Roman Catholic with regard to these essentials of the faith.
Of course, any group that is called a cult based on the above beliefs is free to refer to those who hold the beliefs as a cult. I believe that in Mormonism, the Chrisitianity extant at the time of Joseph Smith was viewed as apostate by the Mormons.
"I have never been able to come up with an objective definition of "cult" other than a religion that a given person doesn't like."
To the Jews and Romans, early Christianity must have seemed like a "cult."