I don't think we are completely on opposite sides on this matter, as I understand and correct me if I'm wrong, that the Eastern Churches believe in the sinlessness of Mary.
Forgive me. I meant something slightly different.
The sinlessness of Mary is one of those areas (as it was explained to me) we have some freedom in our belief. If one wants to believe she was sinless, that is accepted. One can also believe her to be blameless, righteous, in the sense that a number of people in Scripture are said to be blameless. Certainly we do not believe that she had any major sin in her life. But one priest showed me, for example, a writing (I was too new to be able to recall what it was) where she got a bit irritated and impatient with someone at a meeting. That level of sin is no problem for us theologically. And she was, after all, under the curse of death as all mankind is.
The IC is a problem for several reasons, and this is why I said we are on opposite sides of the fence. Most of it stems from the Catholic view of original sin, perhaps. We have some differences in how we view Adam's sin, so we usually refer to ancestral sin. You probably know all of that. But it boils down to Mary having no need to be preserved from original sin (since we don't believe in it), because ancestral sin was not a problem for her to be the bearer of Christ.
More importantly, IF Mary was conceived immaculately, then she is of a slightly different nature from the rest of us. I suppose she would be like pre-lapsarian Adam? In that case, we have concerns that the flesh Christ took from her was not like our flesh. If He did not become one if us then He did not deify the same nature that other men have. So there is no open door for theosis. That would actually make salvation impossible, from our point of view, God forbid!
And then people always ask, if God could simply cause Mary to be conceived without sin, why couldn't He do that for everyone? This is a philosophical conundrum, not a theological point, and as such we are not much concerned with it. But it would seem to paint God a bit more capricious than we know Him to be.
But because of these things, not only do we not believe in the Immaculate Conception, but it is opposed to our theology of salvation. This is what I meant that we are on opposite sides of the fence.
(Yes, we do believe Mary was "full of grace" as the Archangel Gabriel addressed her - she HAD already lived a special life and had been the recipient of the grace of God, and had cooperated with it. And when she was overshadowed by the Holy Spirit, she would have been cleansed, however it would have been necessary, to bear Christ. But the full IC we do not and cannot agree with.)
No offense intended ... these are simply our differences, and reasons for them.