If you sit around thinking about what's the difference between art and not-art, at some point you stumble on generosity.
Of course being damn good at what you do helps a lot. Both Joshua Redman and Brad Mehldau are very, very good musicians. It's nice that they are also great friends and collaborators. Redman has a slight edge in this piece. What's most notable is the generosity of his playing: what he's doing is not about Joshua Redman but about music. That may seem like an odd thing to point out but, in this period of extremes of narcissism, doing something for its own sake comes off as extraordinarily generous.