Beyond the skills and techniques, two things: Empathy and character. Empathy because it is no good being observant unless you can use the observation to put yourself in other people's shoes. And character to impart your viewpoint, to want to influence others; the songwriter's character should come through the song, so that when you hear the song you should get a feel for the person that wrote it. Stan Rogers, Jez Lowe, Eric Bogle, and - most of all - Robb Johnson are good examples of this.