I try to get through these once per week, so one thing I'm very sure of is that E. trads and hymns both involve the repetition of a relatively simple tune (the latter being slightly more sophisticated); both have had chords and base notes added to them (by RVW, e.g.) but, as Treewind suggests, logically, the survivors are the pieces with good WORDS and TUNES, that, again logically, can sound great unaccompanied.