This is by no means a comprehensive list of Mormon beliefs that contradict mainstream Christian views, but these are some of the major ones that leave many a Christian concluding that Mormons are not really Christians.  You make your own decision.

1. Faithful Mormons not only become "like" God, but become Gods themselves
   (D&C 76:50-8 and 95, 132:15-23, 29, and 37; The Latter-day Saint Woman: Basic Manual for Women, Part A, Lesson ...; and Gospel Principles, 302 [1997 edition])
2. Jesus was not born of a virgin birth.  God had literal sex with Mary to conceive Jesus. 
   (mormon-wiki, 1 Ne. 11:18-21; Alma 7:10; The Complete Discourses of Brigham Young 1:321; Young,The Journal of Discourses 1:50-1; 8:115; Heber C. Kimball, 211; Orson Pratt, The Seer, 158-9; Family Home Evening Manual[1972], 126; and McConkie, Mormon Doctrine, 546-7, and 742-3)
3. God, Jesus and the Holy Ghost are three separate physical beings, not one in the same
   (Fielding Smith, The Teachings of the Prophet Joseph Smith, 372 [pre-2002 edition], 
4. Jesus and Lucifer (Satan) are spirit brothers, as are all of God's children
   (Milton R. Hunter,The Gospel through the Ages, 15; Gospel Principles, 15 [2009 edition]; Abraham 3:22-8with Moses 4:1-4; The Pearl of Great Price Student Manual/The Book of Moses 1:12-22 (2000); and Jess L. Christensen, “I Have a Question,” Ensign, 25-6 [June 1986])
5. God is just one God among many God brethren, all relatives in a long succession of exalted offspring who passed the tests of mortality
   (McConkie, Mormon Doctrine, 576-7; Joseph Fielding Smith, ed.,The Teachings of the Prophet Joseph Smith, 346-7 [pre-2002 edition]; Abraham 4:1, Pearl of Great Price; Gospel Principles, 245 [1997 edition], and 302; "God," LDS Bible Dictionary; and Blake Ostler, "Review of The Mormon Concept of God: A Philosophical Analysis by Francis J. Beckwith and Stephen E. Parrish," FARMS Review of Books [Provo, UT: FARMS, 1996], 99-146)
6. Baptism, Confirmation and other ordinances can only be performed by males who receive the priesthood from a succession of authority handed down from Joseph Smith after Peter, James and John restored it to him
   (D&C, Official Declaration--2; 84:6ff.; and Fielding Smith, Doctrines of Salvation 3:80)
7. God has an exalted, perfected physical human body
   (D&C 130:22; 131:7-8; and "Spirit" in the LDS Bible Dictionary)
8. Only those who accept Mormonism in this life, or via Proxy from the spirit world, can inherit the highest kingdom of heaven, aka, the Celestial Kingdom.  All others go to lower kingdoms
   (History of the Church 3:28, D&C 76, Apostle Bruce McConkie, The Seven Deadly Heresies, Spencer W. Kimball, The Teachings of Spencer W. Kimball, 50, and Joseph Fielding Smith, Doctrines of Salvation 2:31, Alma 40:12-4 and D&C 138:30, D&C 131:1-4)
9. The LDS Church is the only church with the fullness of the Gospel, aka, "only true church" 
   (Doctrine and Covenants [D&C] 1:30, Joseph Smith, History 1:9, Pearl of Great Price, Joseph Smith, History 1:19, 1 Nephi 14:10, History of the Church 3:28)
10. The Bible does not contain the complete word of God.  The Book of Mormon, Doctrine and Covenants, Pearl of Great Price and living Prophets are required to have a fullness of God's will and truth
    (8th Article of Faith, 1 Nephi 13:23-42, 2 Nephi 29:3-14, History of the Church3:28)