"Should you ask why we differ from other Christians, as they are called, it is simply because
they are not Christians as the New Testament defines Christianity" (Journal of Discourses 10:230)
Mormonism is not Christianity. FYI -->
Christian churches teach belief in God as an eternal, self-existent, immortal being, unfettered by corporeal limitations and unchanging in both character and nature. In recent years, several Christian denominations have made studies of Mormon teaching and come to the conclusion that there are irreconcilable differences between LDS doctrine and Christian beliefs based on the Bible.
Some Mormons undoubtedly become Christians, and evidently leave the Mormon sect they were members of behind.
(Thanks to delacort for compiling this list)