Sappy music
Well I'm listening to the Titanic soundtrack right now, and that's kind of the epitome of the sappy movie, so by interpolation the music is also considered sappy (Celine Dion? Yeah.). Overwrought emotion makes things "sappy", and I suspect that sappiness is a bad thing because not only do we tend to value emotional restraint, but also because it's supposed to be crudely manufactured to tug at your heartstrings. And people rationalize that, and so don't emotionally respond to it, I think. But I know that some people respond very strongly to instrumental music, like myself, and I enjoy being moved by music -- it would be nonsensical to call that bad taste.