I call music traditional if it's like this:

The average person can sing it (or play it, if it's dance music) - no huge jumps, no crazy intervals, reasonable range.
It has melody.
It has rhythm and rhyme.
The chords to it are not 'algebra.'
    examples of traditional: A, E, D, Bm D7
    example of algebra: Dm7sus6
It has a familiar form: ABBA, AABA, or stanza-refrain

There may be other factors, but that's a start. To me, 'traditional' is not defined by who wrote it, but by the nature of the music.

Greg, you made good points.