What are the biggest misconceptions about Mormons?
That they are so giving and friendly to everyone, even if you aren't Mormon.
Most are friendly as long as there is hope that their actions will give you a positive impression of their church, and maybe some day lead to you becoming a member yourself.
If you reject Mormonism by leaving, the friendliness quickly disappears.
That they do not practice polygamy anymore. Men can be married (“sealed”) to multiple women but women cannot be sealed to more than one man. For example, the prophet is currently sealed to two women. Polygamy still exists as a doctrine.
Mormons are not evil. They do what they do because they believe that they're doing the right thing.