The biggest differences that stood out to me between Orthodoxy and Protestantism are the following:
1. Protestants believe in Sola Scriptura, Orthodox believe in Apostolic Succession and Holy Tradition, of which the Scriptures are apart.
2. Protestants do not have any inkling as to what the Trinity is and why it is important. Orthodox do.
3. Protestants tend to approach theology by defining what God IS. Orthodox use the apophatic method by which they are cautious to say what God IS, and prefer to say what God IS NOT. This avoids putting God into a box...which creates a host of problems.
4. Protestants generally believe that salvation is a legal transaction or declaration of innocence beginning with a declaration of faith or a mental asset to some set of propositions; Orthodox believe salvation is union with God through the process of Theosis (what many Protestants would call "sanctification"), beginning with baptism and chrismation into the Church. The Orthodox do not bifurcate justification and sanctification... it's all under the umbrella of salvation, because salvation means healing, not a legal declaration.
5. Protestants have a juridical understanding of salvation, while Orthodox view salvation as patient/doctor sickness/healing issue.
6. Protestants believe in original sin, which means that humanity inherits the GUILT of Adam and Eve. Original sin somehow alters human nature. Orthodox believe in ancestral sin, which means that humanity inherits the CORRUPTION of Adam and Eve, but that we are only guilty for our own sins. Ancestral sin alters our energies, but not our essence (nature), which was created and remains in the image of God. What was lost was the likeness of God, which needs to be restored.
7. Protestants do not distinguish between essence and energies; the Orthodox do. And the implications of this are interestingly VERY significant. (i.e. The creation of a car... the essence of car is GOOD and remains GOOD. But it can be used either for good or evil, this would be its energies).
8. Protestants say: humans are snow-covered dung. The Orthodox say: humans are dung-covered snow.
9. Protestants often embrace the Penal Substitutionary Atonement doctrine, in which God punishes his son and demands that death be made to satisfy and appease God's justice (again because there is no essence / energy distinction). Orthodox embrace the Christus Victor doctrine, in which Christ did not come to appease the wrath of God, but to defeat sin (by living a perfect life) and by defeating death (by choosing voluntarily to die, hence entering into the realm of death, and destroying the system by implanting the very source of life (God) within death itself and breaking the curse forever).
10. Protestants tend to elevate the invisible spiritual world (soul) above the material physical world (body). The Orthodox believe that to be human is to be soul AND body... and this has implications for everything from ecclesiology and its understanding of the Church, to the way in which we live in the body because what we do in the body matters. (it truly is meant to be a temple for God to indwell).
11. Protestants believe in the "invisible" church, and the plurality of church expressions and ways of worship (relativism); Orthodox believe that Christ leads the visible Church into ALL truth (absolute truth) and has prescribed a way of worship in the OT that carries over into the NT -- the only difference is the passover sacrifice is the Eucharist rather than an animal.
12. Protestants believe that truth comes from the word of God, but that no church can claim to possess the fulness of truth. Orthodox believe that Truth is a person, not a book. This person of Jesus Christ is an objective reality. Therefore, theology centers entirely around the person of Christ (Christology). The Orthodox also believe that the fullness of Truth is experienced within the inner life of the Orthodox Church.
