Musicality" in a system? What IS that ?

I thought I would venture to bring a question in, the interest in which unites us all. What has happened, when we describe a system as "musical"? Is it just a subjective and passing state of mind, which fills us with joy as we listen and if so, what does it need for us to get there? System tweaking perhaps or rahter "ego tweaking" like good company, a good wine, a good cigar etc? Both perhaps? Or could there be objective criteria, which have to met for a system to attain this often elusive and volatile quality? I am convinced that there are...but to your mind, what are they?