i dont get how our beleifs supposedly "differ" from the rest of the world.  
 
very simply, this is what we beleive.  
 
we beleive that god, decided that we were all going to heck if we couldn't repent! so he sent his son, jesus christ, to atone for our sins. (doesn't that mean we believe in christ??) 
 
we believe in the bible! (but that it has been translated a little bit not-as-acturately as it could have been.  
 
we believe in the book of mormon!(simply another book of scripture, except from a different part of the world. it talks about when jesus visited the america's after he was resurected.) 
 
we believe there are also other books of scriputre out there.(where on earth did the 3 wise men come from? they must have their own scriptures and things too, or else how would they know to come to see baby jesus?)

I think not believing in the trinity is the main reason mainstream Christians think LDS aren't Christians. From my experience they view our doctrine of the Godhead (and having the Father, Jesus, and the Holy Ghost as seperate beings) to be polythesistic. 
 
Many many times i've debated this very topic with non-LDS Christians and they tend to use the many bible verses that proclaim that Jesus and the Father are one. 
 
However they fail to see what Christ means when he says that He is one with the Father. Christ Himself explains what He means in John 17: 
 
11 And now I am no more in the world, but these are in the world, and I come to thee. Holy Father, keep through thine own name those whom thou hast given me, that they may be one, as we are.  
 
21 That they all may be one; as thou, Father, art in me, and I in thee, that they also may be one in us: that the world may believe that thou hast sent me.  
 
He is not proclaiming that He is the same being as the Father (it would be silly to believe that Christ is praying to the Father pleading that we may all morph into the same being), but He is proclaiming that we need to be unified in purpose just as the Father and the Son are.  
 
The bible itself is the trinity's worse enemy.

I am a Protestant Christian and the one thing that has confused me about Mormon beliefs is the Book of Mormon. In Ecclesiastes 12:12 it says "Be warned, my son, of anything in addition to them. Of making many books there is no end, and much study wearies the body." Isn't the Book of Mormon an addition to the Bible?

You are misinterpreting what the Bible says regarding this. 
 
Here are a couple of links that helps explain: Link 
 
Link

What is christianity really? Constantine converted the masses to christianity. Did he say one religion was christian and not the other? No. other christian religions were founded. It\'s hard to say which religion is christian and which isn\'t because people all have their idea of what christian is. Just because the mormon religion doesn\'t live to the code of other christian religions doesn\'t make them not christian. They are as much christian as their neighbors. They just have different beliefs and doctrines from other churches.