Well, there are many. I was under the impression you were familiar with Buddhism. Is that not true?
One of the primary differences between Buddhism and Christianity is their completely different view of salvation and the means by which it is attained. Buddhism, like all other forms of idolatry, is a works based religion. Though much of evangelical Christianity and all of the Catholic based forms of Christianity are also largely works based, or at least include works as essential to salvation, the heart of the true Gospel is one of utter reliance and dependance upon Christ alone for salvation.
The differences are so insurmountable that I question why you are asking what it is that they express that is contrary to Christian doctrine. Here are a few that may help you see the glaring chasm between Christianity and Buddhism:
Buddhism is monistic. They deny the existance of the Creator. (Honestly, what could make it more irreconcilable with Christianity than this?) They submit that the world operates by natural law and power rather than divine command. Buddhism denies the existance of a personal God. Christianity, on the other hand, states clearly that there is a personal God and He is the only proper Recipient of worship. Because they deny God's existance, they deny the need for a Savior because we are incapable of sinning against a God that does not exist. They deny the resurrection in favor of reincarnation as does Hinduism, which was the predominant environment in which Buddhism was spawned.
"The Buddha is neither a god nor a prophet or incarnation of a god, but a supreme human being who-- through his own effort, attained to Final Deliverance and Perfect Wisdom, and became `the peerless teacher of gods and men.' He is a `Saviour' only in the sense that he shows men how to save themselves, by actually following to the end the Path trodden and shown by him." - Nyanatiloka Thera, Theravada Buddhist monk, author and expositor
If that doesn't show you some glaring and irreconcilable differences between Christianity and Buddhism then nothing will.
God bless