I am a person who happens to in many ways be a much bigger fan of the theology of the new Pauline order of Mass than of the Tridentine rite, but I am a huge supporter of ad orientem.
It’s really got nothing to do with being towards the people or away from the people, it’s got to do with God. Who, when praying, has never knelt down, as though before God, even if there was nothing in front of them? Who, when praying, has never looked up to they sky as if that is where God was? Even Jesus did these things in the Gospels. The fact is that God has no location (spare the Holy Eucharist, of course) because He is Spirit. When we kneel to nothing or look up to the sky, we aren’t
really kneeling before or looking up to God, but we are showing Him that homage as best we can give our physical nature. While we are not really doing these things before God, at the same time we are, because the intention of our hearts, exercising itself through our symbolic actions, is accepted by and pleasing to God. Further, it enables
us to more accurately and truthfully experience and express our submission to and adoration of God. This is all because it points to the fact that God is above us, beyond us, outside of us - He transcendes us.
The ad orientem posture does the same thing with the Mass, the ultimate act of worship and the ultimate prayer. The purpose of the Mass is to offer Christ to God in Sacrifice. Now the Sacrifice is accomplished whatever direction the priest is facing. Yet to face the altar, with his back to the people, is to do for the Mass what kneeling or looking up does for our personal prayer. It symbolizes, and in a mystical way actualizes, that our Sacrifice is being made to a Being that transcends us and is beyond us. While He is present in the congregation accidentally, as the philosophers would say, He is, in His
essence beyond and outside and transcendent of the congregation. So the priest turns, facing with the people, so that all involved are looking in the same direction, towards God. God is, in His essence - that is, His real, true, most base level of existence - outside of the congregation. All of the people in the church have come to direct their attention outside of themselves, towards their transcendent God, and to offer Sacrifice to Him.
http://www.unavoce.org/tmass.jpg
This is the Sacrifice. All of the people are kneeling before God as the priest lifts Christ up in the air, offering Him to the God that is beyond us all. In the ad orientem posture, the Sacrificial nature of the Mass becomes so clear.
And yet it does not take away from the facing of the people, for the priest
does face them, at the appropriate times. Throughout the entire Mass, there are two directions - towards the people, and towards God. When it is God Whom is being given to, He is faced, and so during the prayers and especially the Sacrifice, the priest faces God. When God is giving to the people, the priest faces them. So during times such as the homily and, especially the giving of Communion, the priest faces the people. And so ad orientem, rather than excluding certain parts of worship - such as the rightful emphasis on the community of the people - in fact serves to
include and emphasize all the appropriate parts of worship. It directs the people towards God when that is proper, when the action is one that ought to be directed towards God, and it directs the people to the community, and to recognizing God’s giving to them, when the action occuring is one that ought to be towards them.