This is not to invite ridicule of any religion but I think that the main reason Mormonism is such a lightning rod is that it is of such recent vintage. Imagine a religion making all sorts of claims after documentary evidence became more reliable. Imagine if Moses' trip up to Sinai were videorecorded, or Mohammed's visions, or Jesus's alleged resurrection. Of course, Joseph Smith did not have camcorders following his every move but at least more is known about his life. We are willing to believe that elderly Middle Eastern men spoke with the deity thousands of years ago, but imagine the audacity of claiming something like that happened in the last millenium in North America. That's just too much!
It's not a coincidence that religion is more believable the further removed in time, place, and accurate reporting.