One big difference is Mormonism is spread by clean cut young people doing missionary work. Islam was spread at the end of the sword and is still a horrifically violent religion. When was the last time you heard of Mormon suicide bomber?
I said that I admired the missionary work done by Mormons and listed other attributes of LDS as well...but what separates mainline protestants and Roman Catholicism is mainly differences in doctrine (i.e. Marian philophy, papal authority). And, IMHO, mainline RCs and protestants have more in common with Judiasm than with Mormans because of the leap of faith one would have to make.