While there are many reasons why Evangelical Christians of all stripes might disagree with Mormon theology, perhaps the most important of these is Christology and the related matter of soteriology.
I would encourage you to read carefully through the statement at the link below by a practicing Mormon scholar, presented at Harvard Divinity School a few years ago. Here is the link—http://www.mormonnewsroom.org/article/what-mormons-believe-about-jesus-christ (in order to actually find this page you will need to do a Google Search of ‘What Mormons believe’ and then scroll down to the entry from 2001 when Millet spoke at Harvard. It has been taken down from the Mormon website).
Please note that these views, as expressed by Mr. Millet are not unusual or eccentric, rather they are typical. While it is true that in some respects, Mormons have more disagreements with Catholics and Orthodox Christians than they do with Evangelicals they certainly have major differences with Evangelicals as well. They could not, for example, in good conscious sign a faith statement that the Evangelical Theological Society might present to them for membership in that society. What are these major differences? Here it will be worth listing just a few in this post:
1) Mormons are polytheists, not monotheists. That is, they believe that the Father, Son, and Holy Spirit are three separate beings, thus denying the essential monotheistic statements of both the OT and NT that God is One.
2) Mormons, thus, not surprisingly, deny the doctrine of the Trinity, calling it an amalgam of Greek ideas with Biblical ideas. Their basic view is that the original doctrine of God and of the ‘priesthood’ and key ideas about sacrifice, and leadership of the NT era were lost, as the church became entirely apostate and needed to be renewed, and that the NT church was not renewed until Joseph Smith came along in the 19th century (who btw, had an interest in Methodism whilst he was in Palmyra N.Y. and apparently took part in some of the revivals in the ‘Burnt Over District’ there in the first part of the 19th century). Mormons see the ecumenical councils which produced the Nicean creed or the Apostle’s Creed or the Chalcedonian creed as in essence contradictory to what Scripture teaches.
3) Mormons believe that even God the Father has, and apparently, needs a body, denying that God in the divine nature is spirit. Indeed they believe that God the Father is an exalted man!
4) Just as they believe that the early church became apostate, they also believe the Bible as we have it is not inerrant or always truthful and trustworthy, even on major issues like Christology, and therefore needs to be supplemented (and corrected) by subsequent prophetic revelation in documents like the Book of Mormon, or even The Pearl of Great Price.
5) in terms of soteriology, Mormons deny the sufficiency of Christ’s death for salvation. They suggest, as the linked article says, that each of us must do all we can and then trust in the mercy of God. In other words, the de facto position is that Mormonism is to a significant degree a works religion even when it comes to salvation.
6) The goal of Mormon soteriology is that we all become as ‘gods’ become both immortal and divine, blurring the creator/creature distinction which was already badly blurred by a theology that suggested that God is actually a sort of uber-human being, with less flaws. One rather familiar teaching is ‘as God was, so we are. As God is, so we shall be’.
I bring this issue up now, because of the general ignorance of the American public about whether or not Mormons are actually Christians or not. If they really embrace the official positions of their religious group, they are not Christians, though they often present themselves as such, for example, calling their meeting places churches sometimes (but notice— no crosses to be found on top, or worn either).
What of course makes this whole deal slippery is that Mormon doctrine is a constantly evolving thing due to a belief in the living voice of prophecy. For example, the head of the Mormon Church in my lifetime corrected what had previously been taught by Mormonism’s original leaders (e.g. Brigham Young) that black people were the descendents of the least favored race of the big three (Shem, Ham, and Japeth), and as such could not become priests in the Mormon church. Not so, any more.