Ping!
First, apologetics aside, this is a good, if limited, exposition of +John Cassian's work.
Second, reliance on the Council of Orange and the Council of Carthage, while certainly a fair thing to do, must be tempered with the understanding that neither of those councils were ecumenical and thus not at all binding on the entire Church. They were completely local councils.
Third, while +Augustine was not proficient in Greek and had only a very limited knowledge of the writings of the Eastern Fathers, or indeed of the New Testament itself save in translation, +John Cassian was fully fluent in both Latin and Greek. I cannot stress enough the importance of this because it goes to the very heart of what +Augustine wrote. Take for example the word "sin". In NT Greek the word is "amartia" which means "to miss the mark".
For Eastern Christians, the "mark" is Christ. Our created purpose, before the Fall, was to become like God. The Sin of Adam distorted humanity (indeed all of creation) to such an extent that we could not progess to fulfillment of that purpose. By the Incarnation, our potential to fulfill our created purpose was restored. Indeed, as +Athanasius the Great wrote, "God became man so that men might thereby become gods". Christ thus is called the "New Adam". Because Christ is the "mark", because in order to attain theosis we must become "like Christ", we need to respond to God's grace, which all the Eastern Fathers agree falls on the good and the evil equally, by living our lives in such a fashion as to "die to the self" and in that process, which is fueled only by God's grace, become like Christ. The Orthodox view of Judgment stems from this. We believe that we are not judged by our good or bad deeds at all but rather by how much like Christ we have become, have we "hit the mark" or not. Our good deeds and bad, by the time judgment rolls around, are quite meaningless, indeed they are merely tools to transform ourselves and thus become more open to grace. As you can see, with that sort of understanding of sin, especially of the Sin of Adam, coupled with the East's understanding of what exactly salvation is, there is bound to be quite a difference between Western and Eastern theology if Western theology fully adopts what we in the East believe to be +Augustine's erroneus concept of sin, again, especially, "Original Sin". Its the difference between failing of a purpose and "insulting" God.