As I said, this was just something that Gregorian chant made me think of, since much of it does not rhyme. I don't think that rhyming in and of itself negates proper Theology. That would make absolutely no sense. I was just wondering if the need to make something rhyme limits the potential Theological vocabulary that can be used, making it more difficult to get the proper Theology across. For example....consubstantial. That rhymes with....monumental.. .sacramental....soup with lentils. If you couldn't find a fitting rhyme for the word, you might try to water it down and just say "the same" to make it more pleasing to the ear.
I think that's a good point though..

and I like Gregorian chant a lot too. This could be the case in some hymns. But - this is just my opinion of course - in many contemporary hymns, it's like they don't attempt at the same type of theology as the older ones. It's not that they couldn't find the words, rather their topics are different to begin with. The older hymns also use more "exalted" language if thats the right word. I'm not saying that all contemporary hymns are the same, - they are not, but many seem to stop at gathering together, being joyful in worship... and don't really go beyond that.
There are people who might really like them and be edified by them. Personally, I don't find that they do much for me, maybe because I come to Mass with different intentions. Who knows. I love the old hymns like "Holy God we praise Thy Name" and "Stabat Mater Dolorosa" or "Let All Mortal Flesh Keep Silence" (which is awesome).
The new hymns are mostly found in Gather or Glory and Praise hymnal, the older ones are in Adoremus (which is my personal favourite) or Catholic Book of Worship II (which contains both old and new).
I think melody is also important... I've always liked the melody of "The Angel Gabriel from Heaven Came", or "Ave Maria", or "Tantum Ergo".
Then there are Marian hymns such as:
Mary the Dawn, Christ the Perfect Day;
Mary the Gate, Christ the Heav'nly Way!
Mary the Root, Christ the Mystic Vine;
Mary the Grape, Christ the Sacred Wine!
Mary the Wheat-sheaf, Christ the Living Bread;
Mary the Rose-Tree, Christ the Rose Blood-red!
Mary the Font, Christ the Cleansing Flood;
Mary the Chalice, Christ the Saving Blood!
Mary the Temple, Christ the Temple's Lord;
Mary the Shrine, Christ the God adored!
Mary the Beacon, Christ the Haven's Rest;
Mary the Mirror, Christ the Vision Blest!
Mary the Mother, Christ the Mother's Son.
Both ever blest while endless ages run.
All of them seem to rhyme
God bless!