nothing is. It has a left-leaning bias that I'd say is overall pretty mild. NPR has less of a left bias than Fox has a right bias. But I'll admit it leans left.
NPR wins still though b/c they cover so much. If I just keep NPR on in the car (which is what I do), I get my basic national and international news, plus arts, philosophy, comedy, science, etc. They cover everything. I love it. It's all I listen to.