Angel Gabriel hails Mary, as though a superior to him creature, and calls her "full of grace." What sort of grace is that? It could be sanctifying grace suitable to make Mary holy and assure her personal salvation. But it's more probably the gift of being the mother of God.
But do you really think that God could've picked a random slave-girl toiling in the fields and by copiously giving her "grace" made her exactly as good as Mary?
Why then the thousand-year-long history of the nation of Israel that culminates in Mary and Jesus? It can't just be the Jewish religion, i.e., the strict monotheism, admittedly an island of sanity among the polytheistic nonsense around it.
It must have had something to do with genetic purification, which was in some part responsible for Mary's natural perfection. God was being a eugenicist, and He used Israel to make Mary well-born, not just rescuing her from original sin, but granting her freedom from "concupiscence" understood as imperfectly ordered or sinful desires.
Mary was not a robot who did not enjoy food or drink or was sexually frigid. For example, she was slated to marry Joseph normally before the annunciation, and so she must have expected to have sex with him and even looked forward to it. Angel Gabriel's visit was a surprise to her. Remaining a virgin was a sacrifice Mary had to make for the sake of a more highly valued good, namely, becoming the mother of God.
We may ask: If Mary, who was born without original sin, was a huge success as a human being, so great in fact that she qualified to give birth to God the Son, and is now a queen of heaven, then:
Why wouldn't God have all of us born without the original sin, thereby making every man a king of heaven, equal to Mary, if absence of that troublesome condition is all that stands in the way of such glory?
If more than that is needed, then how was Mary different from Eve who, while also in the state of original natural happiness, still sinned?
Perhaps just as some angels were created already naturally predisposed to accepting God's grace and some, to rejecting it, Mary was naturally superior to Eve, and where Eve failed, she succeeded. Eve was created easily given to temptation, easily corruptible, while Mary was created much more steadfast. Eve then had a major character flaw. It is Mary's greater natural righteousness through which she persevered.
The conclusion is obvious: Eve was created weak, unlike Mary, because the fall of humans somehow served God's purposes. A world in which there is original sin must then be ultimately superior to a world in which it never occurs. More precisely, a world in which Eve's strength equals Mary's is better from the Father's point of view but worse overall when the Holy Spirit's and and Son's missions are also taken into account.
Consider also that Mary was conceived without original sin, whereas even now those who are baptized in the Catholic Church as infants gestate in the womb while still afflicted with original sin. So, Mary's body must've been exceedingly healthy and beautiful and strong, free from any inborn defects. This surely was crucial to her mission as mother of God. She had to be up for it physically, and that is never a joke, especially in ancient times, and especially when giving birth to God (of all, uh... people).
So, it seems that Mary was naturally a uniquely magnificent specimen of our species.
Whatever sanctifying grace was given to her, to further her own personal salvation, was ordinary, rather than extraordinary, and similar to graces given to would-be saints today.
She was different from other humans not by the amount or type of sanctifying grace bestowed on her, but (1) regarding the timing of removal of OS -- at conception rather than from baptism at birth or as an adult; (2) her wonderful nature and personality, far exceeding Eve.
But I could be wrong on this one; perhaps in Mary both spectacular nature and spectacular sanctifying grace during her entire life were combined.