"I asked my priest," he said, "and he told me that Mormons are a cult."
Setting aside the obvious riposte ("What did you think your priest would tell you, that Mormonism was true Christianity as restored by God to living prophets?"), I think it's worth considering just what we mean by "cult" and seeing whether it applies to the Mormon Church.
Cult as Bad Word
Anti-Mormons use "cult" the way gay activists use "homophobe"--as an ad hominem epithet hurled to try to silence any persuasive opponent whose ideas can't be countered on their merits.
When used this way, "cult" just means "religion I want you to fear so much you won't listen to them." Or even, "religion I want you to hate so much that you will remove it from the list of churches that deserve constitutional protection."
But just as "homophobe" has a core meaning (someone with a pathological fear of homosexuality to the degree that it interferes with his life), so also with "cult." The only reason it works as name-calling is because there really are religious groups that do--and should--scare us.
There are real examples of what we mean by cults: Jim Jones' group that destroyed itself in mass murder and suicide in Guyana, or those sneaker-wearing folks who killed themselves to join aliens approaching behind a comet. And even though the Branch Davidians may not have been as monstrous as they were depicted in the media, they still clearly fall within what we mean by that word.
What do they have in common?
Charismatic Founder. Cults gather around charismatic individuals who are the sole source of truth to their followers.
Exploitation. The leader enriches himself through the financial contributions of the members, or gathers personal power that he uses to exploit members in other ways to benefit himself. If the group survives the leader's death, it remains a cult if his successors continue that exploitation.
Automatons. The members are discouraged from thinking for themselves, and, insofar as possible, are turned into unquestioning "obedience machines."
Withdrawal and Isolation. Perhaps because exploitation and obedience are easiest to maintain when the ordinary world can't offer its distractions and attractions, cults tend to withdraw physically, seeking ever greater isolation. This is often used as part of the conversion process, to keep the prospective member from hearing counterarguments.
It's worth pointing out that there are very few religions of any size or influence that did not begin with a charismatic founder and whose members did not seem, to outsiders, to behave in much the way I've just described. A humble, wise teacher can always be charged with "setting himself up as the sole source of truth" merely because he offers any unusual idea.
The gathering of money to help the poor or pay for meetinghouses or publications can be called "exploitation." The natural desire of converts to live according to the teachings of their leader can look like lockstep blind obedience to those who live a different way. And if outsiders persecute the new religion, it is only natural that adherents will want to band together and get away, if only for a few hours at a time, to be able to practice their religion in peace.
All religions have a body of teachings that becomes a lens through which the believers see the world around them. To those who don't believe, the lens seems to be a distortion of reality--though of course, those unbelievers are merely distorting reality their own way, through their own lens. No one sees reality without passing the data through the lens of their own preconceptions.
All religions also form a community, however loosely organized, of like-minded believers who set the standard of correctness. Whether that standard is rigid or relaxed, those who cross it are expelled from the community and are treated as heretics, apostates, or infidels. Severe treatment of heretics can be found from the lowliest cult to the largest church, from the most rigid sect to "open"-minded, post-religious academia.
You have to get fairly close to a new religion in order to see whether it is acting like a cult or like a religion. Most of those who hurl the word "cult," however, do not bother to get close. And those who do are often so grimly determined to attack that they distort all evidence in order to support the charge.
How Does Mormonism Measure Up?
Joseph Smith was a charismatic leader, but he was murdered 156 years ago. Nowadays, we have leaders who, while sometimes gifted at communication, are rarely of the dramatic, movement-founding type. Indeed, I feel safe in saying that the majority in my lifetime have been rather dull and gray, and they are followed far more because of their office than because of any personal charisma.