First of all, there is a difference between Catholic and Orthodox teaching on this matter, and for the Orthodox, it is a critical distinction. The Orthodox belief is still one that the vast majority of Protestants would disagree with, but I will try to briefly explain it. Keep in mind also that it only came to be articulated in detail in response to what we felt was an incorrect teaching by Catholicism.
As I said to FK, Orthodox teachings on the Theotokos have generally been part of the "inner tradition" of the Church, and not a part of dogma or kerygma. We find no reason to disagree with these traditional understandings, but as with so many things the Orthodox Church does not declare any of them to be dogmas, with two exceptions.
The first dogma is the Virgin Birth, and I think we would agree with that. It was decreed by the 1st Ecumenical Council The fact that in the Creed, she is titled the Virgin Mary reflects the long-standing belief in her ever-virginity (ditto for Calvin), but it is not spelled out as such.
The second dogma is that she has the title of Theotokos, or the "bringer forth of God" or "birth-giver of God." This was declared by the 3rd Ecumenical Council in response to Nestorianism. The Church detected (correctly as it turned out) that because Nestorius would only call her the Christokos (the bringer-forth of Christ), that Nestorius was denying that Christ was fully God and fully man in one person.
If he was the latter, then we cannot be afraid to call her the "birth-giver of God", or even the "mother of God" (although this latter term is little used in Orthodoxy, primarily because it is easy to extrapolate, wrongly, that we believe that she is the mother of the Trinity, or that she has an eternal relationship with Christ that preceded the Incarnation.)
Getting back to the original issue, the Orthodox Church has never taught that Mary was free from original sin. Part of this is because we view original sin and its effects differently. We believe that the ancestral sin of Adam brought death and corruption into the world, and with it the tendency to sin. We'll have to discuss "for that all have sinned" sometime, and why the KJV got the translation right by choosing that particular construction rather than "because all have sinned."
We do not believe that the effects of the ancestral sin include automatic damnation -- that from the moment of conception a human being is worthy of hell-fire and everlasting punishment because of inheriting the moral guilt of Adam.
What this means is that we believe that Mary was born exactly the same as we were -- with the effects of ancestral sin (corruption, death, and the inclination to sin) just as we are born with them.
We *do* believe that she lived a morally guiltless life, and that this made her a worthy vessel for bearing Christ. This is why she is held up as the exemplar of what it means to be a Christian -- a model for all in her purity and dedication to God.
If anything, this perhaps makes the Orthodox view of her more exalted than does the Catholic view from a human point of view. She did what she did with the same tools at her disposal as we have at ours. Had she been super-charged with special grace at conception and born free from original sin in addition, she could hardly be a model for us since the deck was stacked for her, could she?
An interesting point is that the Eastern tradition is clear that Mary died. On the 15th of August we commemorate her repose, not her Assumption (there are hints of an Assumption in our tradition, but it is not spelled out terribly clearly in the services). This is critical for us, since it showed that she fully had the effects of the ancestral sin in her -- she grew old and died. Only because of Christ's death and Resurrection is her own resurrection possible. As you would guess from logical extrapolation, there emerged in the Catholic world a teaching (not at all universal) that takes it to the logical conclusion -- that she couldn't have died, and was just taken up into heaven without dying. How could she have died if she was free of original sin and lived a morally sinless life as well? But the clarity of the Eastern tradition prevented this from being incorporated into the Catholic dogma of the Assumption. Apparently Pope Pius did believe that she didn't die.
Anyway, that's probably more than you wanted to know, and it certainly is something that you would disagree with, understandably, as a Protestant. There is no direct Scriptural evidence for any of this. It is part of the oral tradition of the Church. I'm pinging a couple of the other Orthodox to see if they would state any of this differently.
I believe where most of us Protestants would disagree is with Mary being described as "morally guiltless" and, of course, we would reference that "all have sinned and fall short...". I think Calvin (and Luther) was too close to the Catholic Church on this view coming directly out of Catholicism as he did. His arguments for Mary are a tad bit weak unlike many of his other doctrinal stances.
If memory serves me correctly, while tradition speaks of Mary being a perpetual virgin, I don't believe many of these documents came into being until several hundred of years after Christ and Mary. Her ascension (Catholic doctrine) wasn't talked about in writing until 5-6 AD. While Mary was undeniably a great woman of God and IS blessed, in my mind it is a romantic notion to believe that she was morally guiltless. None of this is supported by the inspired writings of God's word and there are plenty of other scriptures that state otherwise.
However, I will say that while I disagree with you theology, you Orthodox are more consistent with your theological views than the Catholics, IMHO. I appreciate well reasoned theology even if I disagree with the conclusion.
BTW and FWIW-It is my particular view that all women wombs are holy and pure. Mary, outside of her humbleness and dedication to God which is NOT to be minimized, was physically no different than any other woman. Original sin is transmitted through the male, who is corrupt. Consequently any woman could have been chosen as a vessel of an incorruptible seed. The issue isn't the vessel as much as the seed which required a virgin birth.
I wouldn't state it any differently at all, but I want to thank-you for the information about the Latin dogma of the Assumption. I always thought that the Latins too agreed that she had died but dogmatized the pious belief that her body had been assumed into heaven.