While it is true, as Megasam suggests, that some people simply have better (more sensitive) hearing than others, I don't believe the issue is hearing acuity as much as it is being the type of person who is open to the particular set of emotional offerings and challenges that music gives an involved listener. I know some who are as passionate about sports or cooking, for example, as I am about music; but as unbelievable as it may seem to me, when it comes to music, they can take it or leave it. Sure, they may occasionally hum along to a tune on the car radio, but it certainly is not a priority in their lives; as it is to some for whom it is more of a need, a need to listen to music. Of course, it is very possible to develop a better appreciation of music through more involved listening or coaching. These same "tin ears" might be the ones who are capable of recognizing the different mating calls of various types of ducks. But some people are, at the risk of being politically incorrect, emotionally constipated in this respect. After all, good music will move us, make us feel angry, or sexy, make us tap our feet or dance; to some, unfortunately, this is just too powerfull a force to lose control to.
Then you have the listeners for whom the act of listening to music has to be, by necessity, an uncomplicated act; even if all that we are talking about is the possibly imposing personality, to them, of electronic gadgetry. They are content listening on the simplest, perhaps only a table radio, of systems. While there is no doubt that high-end audio lets us hear more of the music, I think it's important for us audio nuts to remember that the truth of the matter is that the truly important point or message of any music usually gets through on all but the worst (distorted) means of playback. Curiously, I find that self proclaimed "tin ears" are usually music lovers; sure they can hear the difference, they're just not interested in all the trappings of high end audio.