Of course, the planner needs to have the appropriate knowledge and skill for the particular task at hand, and that may vary considerably from one job to another. One needs different knowledge and skills for doing urban design than, say, for modeling traffic flow. However, some basic abilities are highly important across the whole spectrum of planning.
One basic skill, which is probably not very teachable, is just being able to understand the political environment around oneself. Planning and politics are intimately related, and people who rise in planning generally have political smarts.
Planning is ultimately about persuasion. Good plans that are poorly expressed and poorly presented tend to end up on the shelf under a layer of dust.2 Therefore the ability to speak well in public—to express an idea cogently and also to respond well to questions and criticism—is extremely important. The planner who cannot do this, if he or she stays in planning, will end up working for the planner who can.
The ability to write well is also extremely important. The planner doesn't need great literary gifts, but it is important to be able to explain things clearly and in a user-friendly way. In both writing and speaking it is important to be able to come in at the right altitude. One doesn't want to write or talk over the audience's heads or befuddle them with technical jargon. But one doesn't want to come in too low and insult their intelligence either. In short, political smarts and good communication skills are important across the entire profession.