One Approved, I see where you're coming from. However, do you really think it's that subjective? As you said, a classical music piece has a lot more going on behind it than your typical song. Sure, it's definitely a matter of preference in the end, but some things apply, whether one likes it or not.
Can you really say, for example, that a song is good just because you like the lyrics? Can you label a thrash metal song bad because you can't see the music behind it, while in fact it can be quite hard to play (yes, I'm a metalhead)? In the long run, there must be some things that make a good piece... things that apply to every good piece.