In all seriousness, I don't think there is a single answer to this question.   Every successful folk radio host that I know has their own unique personality and own taste in music and their own style of presenting the music.

Frank hit on the key ingredient - doing homework. Sharing that information without lecturing the audience is critical. Each host needs to develop their own way of talking to their audiences.

I've heard some wonderful shows that focus on single styles of folk music - some shows are strictly traditional, some are strictly contemporary. Both styles can work if done with taste and personality.

For me, my favorite shows are the ones that show the diversity of styles and cultures that make up our folk community. Sort of like attending a folk festival each week - as you go from stage to stage you will hear different style performers and have a good time.

Personally, I have found that playing requests can be dangerous. You end up sounding like a jukebox and programming your show for the small percentage of people that will call or write you. They are not your entire audience.    The same advice for not playing only your favorites. The show will end up playing the same songs and performers and will get boring for the host and the audience.

Have fun, the audience picks up on that.