First, it must be noted that the reference to "your spirit" is used in greetings by St. Paul in his letters to the Timothy, Philemon, and other places. The new translation of the Order of Mass is therefore more precise and explicitly biblical, and because of this, it is my hope (and indeed the Church's hope) that the deeper realities and meaning behind the sacred liturgy will be better brought out.
Second, the use of "and with your spirit" in the context of the liturgy is quite truly brimming with meaning from the earliest forms of the liturgy. The translation notes feature this Q&A:
Where does this dialogue come from?The reference to "your spirit" can therefore be contemplated as a reference to the indwelling gift of the Spirit, precisely that Spirit poured out at ordination, and precisely the very same Spirit through which the action of the mass is to be performed. The current reference to "you" in the current translation only refers to this indirectly and is therefore imprecise. It doesn't seem to evoke or sustain the spiritual and mystical reality of what is taking place.
The response et cum spiritu tuo is found in the Liturgies of both East and West, from the earliest days of the Church. One of the first instances of its use is found in the Traditio Apostolica of Saint Hippolytus, composed in Greek around AD 215.
How is this dialogue used in the Liturgy?
The dialogue is only used between the priest and the people, or exceptionally, between the deacon and the people. The greeting is never used in the Roman Liturgy between a non-ordained person and the gathered assembly.
Why does the priest mean when he says “The Lord be with you”?
By greeting the people with the words “The Lord be with you,” the priest expresses his desire that the dynamic activity of God’s spirit be given to the people of God, enabling them to do the work of transforming the world that God has entrusted to them.
What do the people mean when they respond “and with your spirit”?
The expression et cum spiritu tuo is only addressed to an ordained minister. Some scholars have suggested that spiritu refers to the gift of the spirit he received at ordination. In their response, the people assure the priest of the same divine assistance of God’s spirit and, more specifically, help for the priest to use the charismatic gifts given to him in ordination and in so doing to fulfill his prophetic function in the Church.
St. John Chrysostom, father of the early church (AD 347–407) preached this in his "Homily on the Holy Pentecost" concerning the expression:
If the Holy Spirit were not in [Bishop Flavian of Antioch] when he gave the peace to all shortly before ascending to his holy sanctuary, you would not have replied to him all together, And with your spirit. This is why you reply with this expression not only when he ascends to the sanctuary, nor when he preaches to you, nor when he prays for you, but when he stands at this holy altar, when he is about to offer this awesome sacrifice. You don't first partake of the offerings until he has prayed for you the grace from the Lord, and you have answered him, And with your spirit, reminding yourselves by this reply that he who is here does nothing of his own power, nor are the offered gifts the work of human nature, but is it the grace of the Spirit present and hovering over all things which prepared that mystic sacrifice.A great deal could be said about the remaining translation revisions. We'll visit those later.