to me a good old song is one that is old to me or maybe even old to the century that makes me feel. What i feel can be motion or emotion or "that's a really cool line" or "love that little lick" or . . . i thinks it very personal. But Songs that i think are good old songs and that i start playing and then perform become songs that other people notice or like, too. Or even don't like. Depends how what they are feeling.
One of the fun things about playing in a band is that similarilties of taste develop so that my mandolin picker can bring me a tape of "some good old songs" and I know that i will, for sure, love them.
And what fun! There seems to be no end to good old songs. There's a ton of them.