If by hymn, one means a song for the entire congregation to sing together, then in my opinion, a good hymn:

should not have a big range
should not be fast
should have an interesting melody
should have something to say
should have lyrics based on Scripture
should not be an obvious imitation of the latest pop hit
should not sound like something played in a bar
should lend itself to vocal improvisation
should be guitar (etc) friendly.
    should not constantly move stepwise
    should not be marked with chords that look like algebra