Of historical interest: from the preface to his hymnal, here is the 19th-century description of “a good hymn” by Roundell Palmer (the first Earl of Selbourne):
A good hymn should have simplicity, freshness, and reality of feeling; a consistent elevation of tone, and a rhythm easy and harmonious,but not jingling or trivial. Its language may be homely; but should not be slovenly or mean. Affectation or visible artifice is worse than excess of homeliness: a hymn is easily spoiled by a single falsetto note. Nor will the most exemplary soundness of doctrine atone for doggerel, or redeem from failure a prosaic style.
Roundell Palmer, ed., The Book of Praise, 2d ed. (London: Macmillan and Co., 1866), i-ii.