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).
Click on the link: