When we Catholics look at the many instances Scripture describes someone as being full of grace, as in the case of Jesus (Jn 1:14), Stephen (Acts 6:8) and Mary, we need to ask ourselves why would the Father equip them with boundless amounts of grace?
Since one of the most trustworthy definitions of grace is “God’s undeserved favor”, we Catholics can confidently state God granted Mary His ‘high favor’ by making her ‘full of grace’ in order she may fulfill the purposes the Father designed for her. And such purposes are raising the Messiah and providing spiritual support and intercession for His Church.
The Greek use of kecharitomene makes it almost impossible to translate the passage in common terms, but that doesn’t mean we can’t infer the idea the author wanted to convey. The word in question is a perfect passive participle in the feminine, better understood as “having been graced” – but it also partakes as a title or a as name, since it is preceded by the archangel’s salutation. So just as we would greet a king: Hail, His Majesty -or- Hail, Rabbi -or- Hail, King of the Jews; it must be understood God wanted Mary to be identified with such a title.
The problem between Catholics and Protestants does not arise so much from what wording is being used as a translation for such verses, but from the suppression of its meaning and whether or not Mary’s role is denied. This is quite similar to what happens when Catholics grant the Rock in Mt 16:18 may refer in different senses to Peter, his confession of faith and Christ himself; yet protestants purposely deny the first one out of a doctrinal bias.