+PiKhristos af tonf! Khristos anesti! Al Maseeh qam! Christ is risen!+
Peace and grace.
Unfortunately, Paradosis, the 5th "Ecumenical" Council is completely wrapped in politics, which anyone honest in their research will come to see. EVEN if it was doctrinal, and we have issues with the Tome of Leo until this day, we could still never accept that council. The Tome is extremely problematic, and is not following the traditional Christology of St. Cyril of Alexandria.
The Tome states:
There is nothing unreal about this oneness, since both the lowliness of the man and the grandeur of the divinity are in mutual relation. As God is not changed by showing mercy, neither is humanity devoured by the dignity received. The activity of each form is what is proper to it in communion with the other: that is, the Word performs what belongs to the Word, and the flesh accomplishes what belongs to the flesh. One of these performs brilliant miracles; the other sustains acts of violence. As the Word does not lose its glory which is equal to that of the Father, so neither does the flesh leave the nature of its kind behind. To the ear of the Orthodox, this expression is defining the WORD as
doing something, and the FLESH as
doing another thing...as if there is some kind of on/off switch on
WHO does what. In other words, to us, it totally looks Nestorian.
Now, I personally do not want to embark on an all-out debate on the validity of the Tome, and I will grant that the current Eastern Churches understanding of it is not heretical, but there is still no way that St. Dioscorus would be willing to sign that document...but he was never excommunicated for that anyway, as Brett pointed out.
He was excommunicated for 'failure to attend' after the thrice-summons - it didn't matter that he was held under house arrest, it didn't matter that his testament at the council was 100% Orthodox and none of the Bishops attending said anything about what he said (one need only take a look at the minutes to see that)...the fact was that the Constantinopole/Rome vs. Alexandria conflict was at its peak, and Leo was not happy, neither was Flavian or his people.
So yes, it WAS political, it was not doctrinal. If it was doctrinal, then like at all 3 Councils before, they would have held Dioscorus up to defend his position on the Orthodox faith, they would have told him the error in his Christology, they would have in Christian
love done their best to convince everyone of the Orthodox faith, but they did not. They did not follow the steps of Athanasius before him, they did not follow the steps of St. Cyril who wrote letters to Nestorius asking him to reconsider, and only anathematizing after Nestorius refused to sign a testament of the faith, and also when Nestorius explained his views (which were heretical)... Even Macedonius was given the chance to defend his position and was told why it was wrong... Not so with St. Dioscorus.
The 'definition' of the council apart from the Tome, is what the Orthodox Church has always believed, but the procedures of the Council were very questionable, and hardly charitable, and it would be difficult to argue that it was truly doctrinal in terms of the decisions made at that council.
In honesty, I believe that we are Orthodox, we have the fullness of truth, and it pains us that the Easterns are not in communion so that we might be whole in our fullness together.

This is a matter that requires much humility on both our parts - and as a Russian Orthodox Priest told me (and he was quoting a Roman Catholic), 'the Road to Unity will be reached on our knees'.
I forgot to add one thing, Copts are by very nature "traditionalists", although, there was nothing ever changed in our church that we had to return to or fight to keep.
Agape.