Why do so many other Christians hate the Mormons?

I am a Catholic, and I have noticed that so many other Christians (virtually all of them Protestants) rant against the Mormons/Latter Day Saints. I have known many of them, and while I find their theology unfathomable, they have always impressed me as a thoroughly delightful people whom I like intensely (does Mormonism make people more decent, or is it that decent people are attracted to Mormonism?). So, why do so many Protestant Christians, especially fundamentalists and Evangelicals, slam them so much? Many of these are the same nut-jobs who tell me that I am not a Christian because I am a Catholic!