What is the difference between Orthodox Christianity and other forms of Christianity?

Oriental Orthodoxy and Church of the East have Christological differences from other Christian churches. The Church of the East (Assyrian) rejects the Council of Ephesus  (Christ–God is the same being as Christ–Man), and Oriental Orthodoxy rejects the Council of Chalcedon (Christ–God is a distinct nature from Christ–Man).

This diagram in Non-Chalcedonianism  helps: Non-Chalcedonianism .

The doctrinal distinction between Eastern Orthodoxy and Roman Catholicism  is the Filioque : whether the Holy Spirit proceeds from (obeys, if you like) the Son as well as the Father.

The substantial distinction between all the branches, of course, is not theological but identity-based: ritual and political.