One thing that is nearly universal in Christianity is that belief and salvation are tightly linked. This is significant when considering the status of any individual in relation to the Christian God.
In order to be "saved" from the consequences of sin, the potential Christian must first believe in and then accept Jesus' death and resurrection as a substitute punishment for their own sin. As such, "What" is believed may be important. If one person believes the sacrifice of Jesus and their acceptance of it is the sole thing that saves them, while another believes that the sacrifice merely shows a way for them to live their life better and more nobly and as such leads them to strive to live their life similarly, and thereby are they saved by their own virtuousness ... those are vastly different beliefs. Both could be said to 'believe in salvation through Jesus', but they hardly believe the same thing. The second doesn't even believe that the death was a substitute, except in the loosest of senses.
When people believe (as most Christians do) that belief is the key to salvation, they must place some kind of boundary on how close one must be to believing the 'correct' thing in order for salvation to 'take hold'. I always think it is bizarre when Christians broaden the accepted allowable beliefs into virtual nothingness. Hell, I (an atheist) could claim to be saved merely because I 'believe' Jesus is a nice concept. In fact, the Christian apologist and fiction writer C.S. Lewis almost said as much in some of his writings.
So, let us examine Mormon belief about that key matter.
Consider the trinity. It does speak to the nature of Jesus in particular... whether he 'is' God or not, so it speaks to the nature of the sacrifice that was made: Was it God who died on the cross, or was it some subordinate being? It could be significant to God for the potential believer to apprehend the nature of 'his' sacrifice correctly... he may want the believer to appreciate exactly what occurred, appreciate exactly what God did.
Consider the efficacy of the sacrifice. This speaks to the level of trust one places in God's works. As such, I think it may be more important than the trinity. There are basically three categories of belief about this issue, and it brings in some other beliefs inherent to Christianity. (Some might say it can be reduced to two levels of belief.) Are you saved by Jesus sacrifice alone, or are you saved by being a reformed sinner, or some combination of the two. Most Evangelicals would claim to be 'saved by Jesus sacrifice alone', but they do not believe the same thing about this as the Calvinists. And I don't see the Evangelicals agreeing with the Mormons.
Calvinists believe that there is nothing you can do from your own will to be saved, whether action or thought. That is, they believe that humankind is utterly depraved and evil and completely unable to pull out from that state simply because that state is so complete that it is self-sustaining. Left to our own devices, we would be utterly miserable and lost in lawless chaos. The only reason why any goodness remains in human thought and deed is because of the influence of God. God chooses to restrain mankind's inherent evil for his own purposes. God also chooses to move some humans to believe in and accept Jesus sacrifice as salvation from their sinfulness, which Calvinists believe mankind is completely incapable of doing on their own. Thus, they believe, God is the sole author of anyone's salvation, and it is not a credit to any person's own virtue that they believe and accept Jesus' sacrifice. Calvinists believe that the miracle of salvation is irresistible: That once it is offered to any person through the ministry of the Holy Spirit, that said person will believe and accept salvation. God does not try to save you, either he does or does not.
Evangelicals (in general... sheesh) believe that belief in and the opportunity to accept Jesus sacrifice is available to everyone, and that it is not irresistible. That the individual is presented with the opportunity to accept, and then can make their own free decision as to whether to do so or not. The fall is not so complete that mankind is so utterly depraved that he is incapable of any good thought or deed. Nevertheless, mankind remains in a state of sin and condemned to separation from God until the individual chooses correctly. God offers a way to shed sinfulness, and transforms the heart of the believer away from sin, but leaves the believer with free will to engage in sin. To the Evangelical, God offers salvation but mankind retains free will to become saved, or not.
Mormons (along with some other protestants, and possibly even Catholics) believe that you are under threat of punishment for "your own sins", and not a result of the fall through Adam. I have always found it difficult to truly pin Mormons down to a self-consistent set of beliefs, so I can only do my best, here. The main point, however, would be to distinguish (or not) Mormon beliefs from Evangelicals. To my understanding, Mormons believe that ones own efforts to be pure garner credit toward one's own salvation. To the Mormon, Jesus sacrifice is not 'enough' to complete the work of salvation. But, they also don't seem to believe that the fall was as significant as others.
The Evangelical would object to this. The Evangelical would say that the Mormon has not truly accepted Jesus sacrifice, just as the Calvinist would in turn say of the Evangelical. Whether "not fully accepting Jesus' sacrifice" is enough for salvation is an open question. However, to simply dismiss such differences in belief, when belief is key to what it is to be a Christian seems odd to me. For liberal 'Christian' churches, this is probably ok, because they don't even believe that belief is 'key' any longer, stating from their pulpits that belief in Jesus is 'one way' to have a relationship with God. But, for anyone who believes that belief in and acceptance of Jesus' sacrifice is necessary for salvation, I would think that this person would think it was important as to the nature (or "true" meaning) of the belief and sacrifice.
Disclaimer: I repeat that I am atheist, and do not believe any of these things. I just find the discussion interesting.