Some good reading material on this:
Orthodox Veneration of the Theotokos
Regarding her life, I would give strong support for picking up the second book that Nicholas mentioned--it's about as complete as one can get in English regarding the Orthodox position on Mary and her life. We Orthodox do affirm that Mary was "sinless," which we understand to mean that (as Nicholas said) she lived her life in such a way that she never chose to sin. Some early fathers didn't hold to this belief (e.g., St. John Chrysostom), but the Orthodox Church, for the most part, affirms her sinlessness.
Justin Kissel wrote:but the Orthodox Church, for the most part, affirms her sinlessness.
But since the Orthodox Church has not declared anything about the Blessed Virgin Mary's "sinlessness" as a dogma of the faith in an Ecumenical council, I am assuming that is open to opinion.
One could say she sined, and not be rejecting a tenet of the Orthodox faith...right?
Well, that's up for debate Personally, I wouldn't agree with breaking communion if someone didn't believe in her sinlessness, but I think that that person would have major problems singing a lot of the Orthodox hymns which magnify her life. Probably many would break communion over such an issue. In Orthodoxy, there is a reluctance to dogmatize things and set them down in perfect and systematic order, as is done normally in western Christian groups. Just because it wasn't dogmatized, that doesn't mean it's up for grabs. Also, having early patristic support doesn't mean something's up for grabs either (e.g., a number of early saints affirmed some form of the chiliastic heresy). I guess in the end--for me at least--it comes down to obedience and faithfulness to the Church's tradition. For me (and I don't mean to judge anyone), if the Church affirms something, then I try to follow it and believe it as best I can, even if I don't really understand it.