The problem with Mormonism is that it is heavily modified christianity.
At the forefront is not Jesus Christ but Joseph Smith and not the New Testament but Book of Mormon. The story of how he allegedly found God's scrolls is just a bit too much for some average protestant or catholic. They also have concepts of how even before you are born you are an adult and had an existence in heaven and how you at the end of it all can govern your own corner of the universe (you can be like optimus prime there).
I almost never agree with some bible thumping nutcases but if you really think about it, mormonism IS a cult.