Yay first post!
Let's put faith aside for a second and answer this academically.
Consider the theology of group of Christians today, whether they be more

Protestant or

Catholic/Orthodox/Assyrian Church of the East. Consider also groups from antiquity and the Middle Ages that called themselves Christian but those on this forum do not generally hold to be Christian, for example--Monophysites, Nestorians, Monatists, Donatists, Sabellians, Arians, Modalists, Albigensians, etc. Consider also groups that some claim influenced or were heavily influenced by Christian thought as it was formed--Neo-Platonists, Manichaeans, the Cult of Mithras, etc. Finally consider some other groups that call themselves Christians that we generally do not consider Christian--Jehovah's Witnesses or Seventh Day Adventists.
Now look at LDS theology. Simply, you guys are so different from any of these groups that have ever called themselves Christian at any point in the past that we know of. Ever. And this is not nit-picking. This is not a dispute over how precisely God gives us free, unmerited gift of Grace. This is not a dispute over hyper-Calvinism vs. High-Calvinism. This is not a dispute over infant vs. believer's Baptism. Those arguments, aside from being minor, generally have analogs in different Christian groups. LDS has entirely different conception of the nature of God and man's relationship to him. LDS believes that God was a man on some planet in some universe somewhere, who was a faithful Mormon, who got to become his own God, and created his own world, and taht Jesus was one of his spirit children that he produced through intercourse with a spirit wife, who was sealed to him during his time on some other Earth. And you guys believe that you too, can become like him, and be Gods of your own universe.
Where do you find anything like that theologically in any other group associated with Christianity? We have to draw the line somewhere, an academically, it makes sense to say that a group with an entirely different theological conception of the divine from anything else ever considered or assocaited with Christian is clearly not Christian.
While LDS does, sociologically fit in with some Christian sub-cultures (in the United States at least), that is not enough to make you guys Christian. At most we can say that sociologically in the United States, LDS can be seen as part of the general movement towards proseltytizing, individualzed faith common amongst Christians.
To be completely honest, I find

Islamic theology close to Christianity than LDS theology. I don't know how many on these forums will agree with that assessment. I think more would agree with me that Jewish theology is closer to Christianity than LDS theology.
To be honest, the more I look at LDS theology, the more it reminds me of Scientology with a Christian veneer.
This is aside from reasons of faith, where many of us, well, we'll just have to agree to disgree on this one.
Harsh as this sounds, I really do genuinely find LDS member fantastically easy to get along with, and I admire their Pro-Life standpoint

.
Also I hear they use smiley faces a lot.