One thing that probably helps a hymn is if its syntax isn't the sort that has been unpleasantly inverted by linguistic evolution. (Can make you empathize with the linguistic creationists.) A good example is Watts's "Who Shall the Lord's Elect Condemn", the first line of which now appears to mean "Whom shall the Lord's Elect condemn?" when it actually meant "Who shall (or Who's in a position to) condemn the Lord's Elect?"

Haruo