Today I attended a Coptic liturgy, and I was incredibly impressed. The hymns, the deep reverence shown by everyone in the church, the prostrations, the beautiful icons, etc... it was all beautiful and profoundly respectful and reverent. The service was in Coptic and Arabic so I had to read the English translation in a booklet given to me by the priest, but I noticed how similar it is to the Eastern Orthodox Divine Liturgy of St John Chrysostom, and how similar they are theologically. As an Eastern Orthodox Christian I could participate int he Coptic liturgy without once thinking "hmm, I don't really believe that" or "that's not how we see it in the Antiochian Church..." - There doesn't seem to be anything in the Oriental Orthodox religion which is incompatible with Eastern Orthodoxy - sure, there are some notable differences (such as the Oriental aversion to the term 'Theotokos'), but it hardly seems like anything which can't be reconciled as cultural and traditional diversity, rather than them having a different faith.
It seems that the heirarchs of many Eastern and Oriental Orthodox churches are actively seeking to promote harmony between the two churches, and joint statements issued over the last few decades have shown that the theological misunderstandings which originally caused the split have been resolved. When I attended the Coptic church today I told them that I was Eastern Orthodox, and they were extremely welcoming and friendly, and in discussions with the priest and several laypersons it was clear that they regarded the Eastern Orthodox churches as legitimate, simply not in ecumenical communion with the Oriental Orthodox churches.
So, if the big problems are resolved and we all seem to be agreeing and getting on nicely, what is stopping the ecumenical communion which would reunite the Orthodox churches? I'm curious as to what problems remain that are so significant that they prevent the Eastern and Oriental churches from recognizing and acknowledging each other's sacraments and legitimacy as branches of the one, holy, catholic and apostolic Church.