The responses here will tell you a good deal about the areas of concern for this candidate. Specifically this is not just about the things they see as challenging, but more so in the way they state it and how they embrace the challenge. Someone for example that talks about the response in a tone or manner that implies concern would be more negatively viewed as a candidate than someone who sees the challenge before them as something positive, something to strive for and tackle. Listen to the tone. Listen to the responses — and probe away.