I'm pretty sure it was Tchaikovsky - although it may have been one of those other old guys - who was fond of saying:

"You can't play with feeling if you don't know the words."

I think, considering the kind of music played by those old 'uns, that he was referring to playing the melody with feeling.

With some experience in playing in a dance band, when people actually danced together instead of this "solo thrash and jerk" stuff that seems popular now, I would venture that good dance music has to "sound like" it's at a constant beat, but if you don't play it with tempo variations that fit the music, people fall down.

John