The crucial question is what is the Mass?
St Paul gives the answer, "When we eat this bread and drink this cup, we proclaim your death, Lord Jesus, until you come in glory" 1Cor 11:26.
The Church, from its very beginning until the 1960's, zealously defended this understanding of the Mass over the 'fraternal meal' as its core teaching and consequently its understanding of the Church's life and mission.
Christ's death and return, and our place in it, is at the very heart of Catholic (and Orthodox) belief, it is this after all that is the essential part of the kerygma. It is this that page for page is the substantial part of the Gospels. It is this rather than Jesus' teaching or life that concerns the writing of the Apostles, and the writers of the sub-Apostolic age.
It is with Protestant worship in the sixteenth century where the idea of the 'fraternal meal' takes over from the proclamation of the Lord's death and return in glory. Protestantism has a supreme discomfort with the notion that the Mass is a supernatural event. In the Catholic Church the 1960's brings in an understanding of the Mass that is essentially one of the 'meal'. It would obviously be foolish to suggest that the Eucharist is not set within a meal but for the New Testament and the Fathers, in fact everyone up to VII, this meal is the sacred act of the Pasch, a communion sacrifice with him the Lamb and Victim Priest.
The loss of a sense of the Mass as being about Christ's death and coming again has been the most significant change in both the Church's understanding of herself and her mission. It would be ridiculous to suggest that this change is not signified by the 1960s re-orientation of the Mass and it is for that reason that Cardinal Sarah speaks about the return to the ancient (and correct) orientation of the Mass as being both 'urgent' and 'necessary'.
To suggest that ad orientem and, errr..., contra populum worship are equal I would suggest is without foundation, certainly ecumenically and historically. The Temple was orientated to the East, there are countless reference Salvation coming from the East or with the dawn, the archaeological evidence, the Tradition of all the Eastern Churches all point to the norm of eastward facing as being normative for Christian prayer both liturgical and private, Msgr Pope in a useful article here reminds us of the care for proper orientation in the Didiscalia written around 250 A.D.
Now, in your gatherings, in the holy Church, convene yourselves modestly in places of the brethren, as you will, in a manner pleasing and ordered with care. Let the place of the priests be separated in a part of the house that faces east. In the midst of them is placed the bishop’s chair, and with him let the priests be seated. Likewise, and in another section let the laymen be seated facing east. For thus it is proper: that the priests sit with the bishop in a part of the house to the east and after them the lay men and the lay women…Now, you ought to face east to pray, for, as you know, scripture has it, Give praise to God who ascends above the highest heavens to the east…What is significant is that the correct orientation of worship was important for our forefathers. It was not arbitrary, one orientation or another was not a matter of choice, as it is still not in the Churches of the East today. One of my Orthodox friends seriously regards the Catholic Church as being protestant simply in regard of abandoning the the ancient orientation. For the Eastern Churches it is a serious issue, and not one of mere preference.
Westward or contra populum worship can best be seen by its fruits, the first and foremost is to diminish the crucial question of what the Mass is about: the proclamation of Jesus' death and return in glory, that is unlikely to be the answer coming from most Catholics today, despite mouthing it in the 'Mystery of Faith' after the consecration.
If we get that wrong then our ecclesiology is bound to collapse, because of course the question raised by lay participants is, "what are we doing here?" If the answer is that we are guests at a fraternal meal, then that in itself raises questions, when there are far more fraternal fraternal meals around than a Catholic church on a Sunday morning. Of course what it marks is a departure from Scripture and Catholic Tradition
For the priest too there is more than a little danger, certainly he has always been seen as alter Christus but he has always seen in this role (literally) on the side of worshippers, like them in all things but ontology, rather than a stand in, or even replacement, for Christ the host of the meal. With the magnification of the priestly role comes also the exultation of congregationalism. Cdl Nichols warns against "personal preference" and yet the whole notion of forgetting the Pauline understanding of the Mass and raising the role of the priest means that individual congregations develop their own style of worship, their own musical tradition, their style of ministry, their own set of preferences, and ultimately their own theology.
Some priests reject the celebration of ad orientem worship based simply on the fact that their congregation would re-act against it. Could it actually be that they have already developed their own congregational theology that is at odds with the Catholic Church. The Catholic Church also includes the Eastern Rites and has a history that goes back through the centuries to Jesus Christ and the apostles. Not understanding the significant fracture that non- ad orientem worship introduces is itself a sign of seriousness of our break with the past and the depth of the hermeneutic of rupture.