In the book "Zen and the Art of Motorcycle Maintenece", the main character is a professor at a university. He's trying to define 'Quality' so he reads a bunch of the students papers in class and asks them to vote on which ones have 'Quality'. The class is unanimous in picking out the best work, or high Quality papers. Everybody agreed that it was obvious which works stood out. So far, so good.
So, the professor gives them an assignment: write a paper that defines "Quality". Rather quickly, the whole class hates this professor and goes into a huge funk. Everybody knows what it is, but nobody can define it.
The professor goes on to try and define it, using it as a means to merge the chasm between philosophy and science. In this pursuit, the professor finally has a complete nervous breakdown and is institutionalized.
I only use this example because I think it has a bearing on this discussion. When each of us individually hears something that we think is great, we consider it to have high quality. We've called it a wildcard, or "it" or magic--whatever. However, like the class, I doubt that any of us can explain WHY a certain musical work has the effect it does. We call it genius or we call it great, but all we know is that it does something powerful to us. Its like the shaman threw the right magic dust (not Cheech and Chongs magic dust!

LOL!)at us and created a miracle.
To me, thats what great music is: a miracle. To call something great is one thing. To try to explain why it is great is impossible. Nobody can lay it out logically and tell us how to create similar miracles in scientific terms. Don't worry, Macle. It won't ever happen.
That doesn't mean that we don't see or hear the quality that exists. I'm sure that some are very gifted at honing in on which songs will strike a common chord. Still, nobody will ever come along and say--"here is how you create a song that the whole world will love--guaranteed. Having the numerous musical styles that now exist furthur complicates the process.
I think the value of all this talk is for the musicians and songwriters. Don't bother trying to define greatness, but get used to what it feels like and sounds like when it happens as an event. Strive to then evoke the same magic in what you create. Even if we can't define it, we know what resonates as being good within us. I think thats about all you can do.
Now, I think I'm done with this, but I want to say--I really enjoy all the different perspectives and ideas. To me, its really interesting and enlightening.