What makes a good liturgical hymn?

What makes a good liturgical hymn? 

A friend recently asked me what makes a good hymn? I had to reply: it all depends on what the hymn is used for, what is its context. I can, however, say what makes a good liturgical hymn, a hymn used in the context of the official liturgy of the Church, especially the Mass and the Liturgy of the Hours. What Sr. Mary Gonzaga OSU wrote about the collect applies with equal force, maybe even more force to liturgical hymns: 

"All these devices of the art of language are necessary for us, for they enable us: (1) to grasp clearly the lessons embodied in the Prayers (docere); (2) to make these lessons more acceptable to us through the charm of diction and structure, in a word, through their appeal to our aesthetic sense (delectare); (3) to persuade us (movere) to mold our conduct in accordance with the principles of faith set forth in the Prayers. This explains  why rhetoric is, and must be, found in the liturgy: it is to dispose us to pray "ut oportet," as we ought to pray.'"

from Rhetoric in the Sunday Collects of the Roman Missal