Part of me hesitates to even post this, because I don’t care too much about this debate. Mormons want to call themselves Christian, and I don’t really care. There is no doubt that Jesus Christ plays a central role in their religion, so it makes sense for Mormons to self-identify as ‘Christian’. Of course, the whole debate hinges on the definition of Christianity, and there are other competing definitions of Christianity. On those more robust definitions (if we accept them), then Mormons are not Christians, though I think it is probably better to just say they are not ‘creedal Christians’. But, as I say, I don’t really care too much about quibbling over who has claim to the name ‘Christian’.
That said, I have very little doubt (and most of the LDS folk I know agree) that Mormonism is very different – essentially and fundamentally different – than every other brand of Christianity we usually think of. Quite simply, Mormon Christians and Creedal Christians disagree about the nature of God (Trinity, Jesus) and the nature of man (whether man is of the same species as God, whether man can become a god). Which is to say they disagree about the most important things.
My only complaint is when some Mormons diminish those differences, acting as if Mormonism was just ‘another denomination’ (as if the differences between a Mormon and a Lutheran are not any more extreme than the differences between a Lutheran and a Methodist). To my mind this is, not to put too fine a point on it, just plain dishonest. (Let me stress, not all Mormons intentionally diminish these differences, but just that some do).
Anyway, First Things has an article on the matter here. Bruce Porter (a member of the Quorum of the Seventy) writes on one side, Gerald McDermott on the other. I think it is pretty fair on both sides (though I am sure my Mormon friends will disagree with some of McDermott’s arguments). You can also listen to interviews with the two authors here.