What makes music great is hard to answer. Was it Liszt who said, “It is easy to say that you like a piece of music, but very difficult to explain why a piece of music is great.” If it is difficult for a composer with more than a passing knowledge of the notes on the page, it is even more difficult for those unschooled in musical theory to say what it is that makes one piece of music appealing and another offensive. But that has not stopped us from prevailing upon others with our opinions as to the greatness of one piece of music and what lacks in others.
Some have tried to explain musical greatness but their explanations tell more about the author than about what it is that makes music great music. We all know that some compositions and some composers are not long remembered and others not easily forgotten. For every one whose name we know, there are dozens whose names nobody knows -- even if they were considered great in their own time!
If you shift this slightly to the subject of hymn tunes, it becomes even more difficult. Though some would suggest that the easier the tune, the more beloved it is, the fact is not easily established. Some of those hymn tunes that are most beloved are also most difficult to sing. On the other hand, there are many easier tunes that nobody cares for. I might suggest that it seems that the more a hymn tune is tied to a musical style in vogue for a time, the more likely it is to be forgotten as tastes move on. But there are tunes, very well connected to the moment in which they were composed, and yet they pass from generation to generation with great affection and are sung with great gusto.
Of course, all of it is clouded by the words. Hymn tunes survive in our memories and become beloved because of the words we sing to them. Take Silent Night, for example. Who can separate the tune from the words? Can it be done? I pray that it will never. Yet some tunes steal away the original words they were meant for and become wedded in our minds to another set of words. Some tunes start out with one set of words and then take on a different meaning with a new set of words later. Here I think of the hymn tune Ebenezer and how it has come to be associated with the great Franzmann text "Thy Strong Word." Others are so attached to a person or moment in history (think here "A Mighty Fortress" that it is hard to conceive of another text which might fit the wonderful melodies (isometric and rhythmic).
It is probably not a good place to start by telling folks why their favorite hymns are lousy choices. In fact, it is probably a good place to start a war by denigrating the hymns beloved to one generation or ethnicity or family. But it might be good to explain why good hymn texts and good hymn tunes have the magic of developing a single identity, message, and persona so that we cannot think about the one without the other. A love for hymn singing, however, is something that does not require great musical expertise to teach or learn and it is with you for a lifetime. While some might think that hymns detract from the faith, I rather think that hymns keep the faith alive as the tune causes the words to form in our minds and upon our lips as an almost unconscious act of faith, worship, and praise.