How do you explain/defend to non-Catholics why Catholics make such a big deal about liturgy/posture/music during mass?
You take them to a very reverent liturgically correct mass and let them feel the awe of reverence and the holiness that surrounds them. If they still aren’t convinced… take them to a clown mass and compare the two.
I think our pope, as Cardinal Ratzinger, explained the matter as well as anybody:
"I am convinced that the ecclesial crisis in which we find ourselves today depends in great part upon the collapse of the liturgy, which at times is actually being conceived of etsi Deus non daretur: as though in the liturgy it did not matter any more whether God exists and whether He speaks to us and listens to us.
“But if in the liturgy the communion of faith no longer appears, nor the universal unity of the Church and of her history, nor the mystery of the living Christ, where is it that the Church still appears in her spiritual substance?,” he asked.
Too often, Ratzinger lamented, “the community is only celebrating itself without its being worthwhile to do so.”
- From My Life: Remembrances 1927-1977
For liturgy try to read Scott Hahn’s book “The Lamb’s Supper: The Mass as Heaven on Earth”. It’s a very good book. For starters try to analyze the earlier chapters of Revelation as the backbone of the Eucharistic Liturgy.