Rather than issue fatwahs against other faiths, I always come back to the basics: the Bible + "something else" (traditions, new revelations, radical reinterpretations, etc.) is simply problematic. Mormons run the risk of having accepted "a gospel other than the one [Paul] preached," and therefore risk being "anathema."
I don't feel qualified to draw conclusions about what the fallout is for non-Biblical beliefs that may end up on top of Mere Christianity, but whatever they are, they can't be good, in this life or the next.
Consider this excerpt from the CHRISTIAN APOLOGETICS & RESEARCH MINISTRY web site:
"Christians are saved from their sins and judgment by putting their trust in Jesus for the forgiveness of their sins. But, faith is only as good as the object in which it is placed. The Mormon Jesus is not the one of the Bible, even though they call him Jesus, say he died for sins, and was born in Bethlehem. The Mormon Jesus does not exist. It is the nature of Jesus that is the issue. Jesus must be God in flesh, (second person of the Trinity) not "a" god in flesh who is the brother of the devil. He must be uncreated, not created. He must be the creator (Col. 1:16-17). This is who the true Jesus really is: God, creator, uncreated, not the brother of the devil. Mormon theology teaches that god used to be a man on another planet, that he became a god by following the laws and ordinances of that god on that world, and that he brought one of his wives to this world with whom he produces spirit children who then inhabit human bodies at birth...."
For the full text re: http://www.carm.org/lds/lds_christian.htm
I'm sure it has nothing to do with Mormons claiming all other churches to be abominations; that and the over 4,000 changes to the most perfect book ever written.
The Mormon church used to teach that it wasn't Christian.