First readability, which has to do with the level of clarity in one's writing. Next would be courage, which is a writer's willingness to "go against the grain" (in terms of what they believe is expected of them) and instead explore taboos or express ideas that they believe are important or interesting. Last but not least would be creativity, by honing one's craft and becoming adept in a number of techniques an author enriches their writing and can deliver a more engaging experience for the reader.
