What are the major differences between evangelical Christianity and Mormonism? I'm looking for concrete theological beliefs and practices on where they differ. Christians say that Mormons aren't Christian and yet Mormons call themselves Christian. So it can be quite hard to tell for a everyday person who is who and what is what.
The "Evangelical" movement spreads across an incredibly wide range of beliefs, so it will be hard to nail this down, but a few things come to mind that ought to have broad acceptance and make at least some form of answer possible. For convenience in the rest of this answer, please treat the word "Evangelical" merely as broadly representing "Most Evangelicals".


Mormons believe men can become Gods, or become like God on this earth. Evangelicals believe God is the singular, unique, and supreme being, and any human transcendence takes place after death or Jesus' return.
Mormons see Jesus as a separate entity from God the Father. Evangelicals view God the Father and Jesus as parts of the same entity. It's hard to understate how important this point is, especially in light of the first point: the Mormon view withholds from Jesus some part of God's place as the unique divinity.
Mormons believe a prophet lead a group of Jews out of Jerusalem to the New World (America) around 600BC, where for a time they flourished. Evangelicals do not.
Mormons believe a prophet to that group created a certain set of gold tablets, which Joseph Smith later translated. Evangelicals believe these tablets are utter bunk.
Mormons believe Jesus incarnate visited the North American Continent. Evangelicals do not. 
Mormons believe in the writings and teachings of Joseph Smith, that they are related directly or almost directly from God. Evangelicals believe Joseph Smith to be somewhere in between a deliberate con-artist, charlatan, and mentally unstable, and point to serious concerns over the credibility of Joseph Smith's claim to 8 witnesses.
Mormons believe the Apostle John is still alive. Evangelicals (with some exception) do not.
Yes, Mormons have allowed and supported polygamy. Evangelicals, at this time, do not, but there are places in the world and times in history where this does not hold.
Mormons believe in something called Pre-mortal Life. Evangelicals do not.
Mormons practice Baptism for the Dead. Evangelicals do not.
Mormons practice a ritual known as "Endowments". Evangelicals do not.
Mormons believe all men will come to glory, but non-believers will come to less glory than practicing Christians... but there is no hell. Evangelicals believe in eternal punishment for non-believers and all those who refuse to submit to the will of God.


The root question here, though, is whether or not Mormons are themselves even Christian. To understand this, you need to look at it from both sides. In a broad sense, for our purpose here one is a Christian when one acknowledges the person of Christ as their creator, savior, and Lord. Everything else &mdash; all the denominations, splits, and arguments &mdash; are just details. 

With that in mind, the Non-mormon Christian perspective is that Mormons have been lead astray, and have allowed the teachings of Joseph Smith and his successors to supersede and in many cases replace those of Christ... and so they no longer follow Christ supremely, but (perhaps unwittingly) follow the teachings of Joseph Smith instead. Moreover, since Mormons refuse to grant Jesus full unity with God the Father, but hold him separate from God the Father, it is offensive to mainline Christians to watch Mormons hold up the teachings of Joseph Smith above or before the level of Christ, yet still use the name of Christ to describe themselves. 

The Mormon perspective is Joseph Smith's teachings are Christ's teachings; Joseph Smith was merely His mouthpiece. They hold both the Old and New Testaments to be scripture (just not all scripture). To be considered "not Christian" is offensive to Mormons, because they still believe Christ died for their sins, just as "regular" Christians do.