From the Greek Orthodox Church in Australia's Website

Another dogmatic difference deals with the person of the Holy Theotokos Virgin Mary. Up to 1854 the two Churches did not differ regarding the person of Saint Mary. Both Churches know Mary as the Theotokos, the Ever Virgin, the Mother of Life, the Queen of the Angels, and by many other titles similar to these expressing reverences for her holy person. In 1854, however, at a Council of the Vatican, the Western Church proclaimed that the Virgin Mary was born without the Original Sin; that is, free of the sin of Adam and Eve. This doctrine was named "Immaculate Conception".

The theory of the Immaculate Conception is based neither on Holy Scripture nor on Sacred Tradition. None of the Fathers of the Undivided Church or any of its renowned theologians ever supported this theory. Even theologians of the Western Church, like Thomas Aquinas and Bernard, opposed this teaching.

The question might be asked, "What harm was done by declaring that the Theotokos Mary was free from Original Sin?" Was it not promulgated out of reverence? One might answer, "Yes", but is it reverent to destroy the unique meaning of the Incarnation of our Lord? This is what it brought about by the teaching of the Immaculate Conception, for, since it was possible for someone to be born without the Original Sin then, in a sense, the Incarnation of our Lord was limited in its general scope. No wonder the Fathers of the Church could not conceive and accept a theory of this kind, for it contradicts the uniqueness of our Lord's Redemptive Work. Poetic and rhetorical expressions found in the writings of the Fathers concerning the Blessed Virgin Mary cannot be regarded as Patristic proof in support of this dogma.

Reverence has its limits, especially in this case where the doctrine of the Immaculate Conception raises the person of the Blessed Virgin to the state of having preceded Christ in sinlessness. Thus, the Virgin Mary is deified. On the basis of this error one might expect more acts of deification by the Roman Church, let us say, of certain saints.
For these reasons, our Church teachers, as did the Western Church until 1854, that the Theotokos Mary was born with the Original Sin but was cleansed by the Holy Spirit at the event of the Annunciation. In other words, the Orthodox Church looks upon Mary as a human being who was purified and subsequently gave human flesh to the Son of God. She is, therefore, called All-Holy (Panagia) because she was chosen to become the Mother of God, a unique vessel of Divine Grace.

For the Orthodox World she will always remain the All Holy (Panagia), the most perfect human being, commanding our respect, our love and our devotion. We will always refer to her as Mother, as Mediatrix, as Purity itself, as the Ever Virgin, as Bride Unwedded. Only a person possessing such tributes could ever house the Eternal Word and Son of God. We neither worship nor deify her. However, since she is the Mother of Christ, we beseech her to help in times of affliction and trial. As the Mother of Christ she is in a position to pray for our salvation, for she is closest to the Lord.