musicians often get asked,
"what's the difference between good and bad music?"
I think this question presumes that music can be described as good or bad; I don't think I agree with that premise. Look at music from two sides: one side contains all measurable materials - some presence of melody, rhythm, harmony, timbre, form and all the rest of it; on the other side is music's effect on its environment - which I'll limit to the subjective experience of the listener.
I believe music can't exist without both sides, just as there is no such thing as a one-sided coin. This is to say that music exists in the subjective experience of the listener as it does in the measurable materials of the composition and performance. It's easy to agree that there is no such thing as a one-sided coin; but to prove music is not one-sided, first think about the old question: 'If a tree falls in a forest and no one is around to hear it, does it make a sound?'; in other words, "Esse est percipi" or "to be is to be perceived." First, I disagree with the idea that something can exist without being perceived by something else, that is, nothing in reality exists in a complete vacuum - even a tree falling unnoticed by someone in the forest is perceived by the soil it fertilizes. As for music, it relies on air and vibration, so music or sound can't ever exist in complete nothingness. (Read Hazrat Inayat Khan's 'The Mysticism of Sound and Music' for some provoking thoughts on that.) Back to the question: "What's the difference between good and bad music?" Good, bad, better, or worse don't refer to morality here. What could they mean in this context then? Is some music superior?
There's no question that the measurable qualities of music can be developed to different degrees. For example, compare 'Spring is Here' by Richard Rogers to Stravinsky's 'Rite of Spring': objectively, Stravinsky's work develops "more" materials of music: more melody, harmony, rhythm, and form; Stravinsky had the aptitude to arrange these elements in a highly detailed, and creative configuration. If we're judging music based on one side of the coin, that is objective evidence to support why Stravinsky's piece is superior to Rogers' - but doesn't that presuppose that the materials of music are an end to themselves? On the contrary, isn't music experienced internally, emotionally? As soon as those 'materials of music' enter the ears of a listener (the other side of the coin), the music engages the listener's elusive cognition of imagination, memory, creativity (to name just three); the sound is transformed neurologically into some subjective sensation... feeling - these feelings are entirely personal, and impossible to measure. Further, these feelings can be dependent on mood, time and space; therefore, music's subjective meaning is never constant, even within one person. Without any means to measure the personal effects (even for one instant), it's impossible to gather a representative affect of a piece of music to measure it against another.
It's a fact that many different people feel passionate about very different types of music. A simple example is, does 'Nessum Dorma' by Puccini has a greater or lesser emotional impact on one person on one day, than 'With or without you' by U2 to another person on another day? Who's to say for sure because the point of view hangs partly on a subjective judgment that does not involve objective evidence. For that reason, I'm not willing to insufficiently represent a definition of music to attempt an answer at the original question. Personally, I think this is a more revealing question: "Why might one ask about the difference between good and bad music?"