The time travelling thing, spoken of so eloquently by Art Thieme and Amos above, gets it about right. The other night a friend sang a version of the 'Flash Lad' - an Eighteenth Century ballad about a thief who 'robbed Lord Golden, I do declare, and Lady Mansfield in Grovesnor Square' - and gets hung, on Tyburn Tree, for his pains. And I swear I saw a ragged lad running through Covent garden vegetable market - scattering cabbages, onions and apples in his wake - with 'Ned Fielding's Gang' of thief takers in hot pursuit...I don't get anything like that from hip hop or thrash metal or whatever - those are just intrusive noises to me - not time machines, not vehicles which link me to my ancestral past.
It's experiences like the one described above that keeps me interested in trad. songs - why I have to learn and sing them myself and why I love to hear them sung well. And, you know, the experience just seems to get better and more profound as I get older - every time I think I've got the measure of these songs I hear tune or a new set of words, or read an article (or a posting here - usually by Malcolm Douglas - thanks, Malcolm!) that blows my mind and sets me off on another journey of discovery.