Well. If you believe that then whats all this stuff about the immaculate conception? Where Mary was born without sin?
The Immaculate Conception is a more recent RC doctrine, formulated since they split with the Orthodox. I'll try to give you a more complete answer later today, but the short answer is that we don't agree with the RC's about their interpretation of inherited guilt for Adam's original sin. We don't believe in any kind of Calvinistic doctrine of "total depravity."
We believe that Mary shared with all men in the sinful state of the world, but that at her core she like all men also preserved the image of God. Since we don't understand sin the way that the RC's do, we don't need a doctrine that - to us - seems to make Mary in to something different from all others.
This is a big topic, and is connected to things like the fact that the Orthodox don't believe in purgatory and that the Orthodox don't believe that an un-baptized infant is eternally condemned.