No one has really said **write songs**. It won't make you a gen-you-wine 'folkie', but if you OWN some good songs...(did I mention that you have to write good ones?)... it can increase your value a lot. If you also do some 'folk' songs, it can be a pleasant mixture.
   The other thing that *I* look for is someone who is knowledgeable and outgoing/gregarious about sharing what they do & know when they perform. That intangible thing called 'stage presence' helps also...I'm not sure one can just adopt that....it''s gotta come naturally.