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.