"What is the highest-impact problem you are capable of solving?"
^^ I think this question is more powerful and much more difficult.
Learning your limits is incredibly hard. You have to border between optimism and pessimism -- between ego and humility.
After you understand your capabilities, follow your curiosity to find great problems. I think the discovery of good problems is itself the important trait. If you rely upon others to tell you which problems are important, you will spin directionless without understanding the underlying principles.