I think you've touched on a few great points, particularly changing tone to match that of the audience. Good list.
I thinks Creativeness is the most important characteristic of a great copywriter. So the ability to write things creatively is the most important skill.
Every good writer should have three skills in him: 1.Capability to put imaginative ideas on paper. 2.Should have good Writing Skills, with good grammar obviously. 3.Should be able to present same idea in different ways, based on the target audience.