It is my experience that the more deeply I learn about a domain, the more questions I have. Similarly, when I know very little about something, I have lots of questions as well (although these are usually much different questions than the questions I have as I gain expertise). In the middle somewhere, the number of questions I have drops off as I mistakenly believe that I understand the domain better than I actually do.
Being able to ask questions about a domain is one of the ways one learns about it. So this diagram implies that a challenge in becoming an expert in a domain of knowledge is that eventually you know too much, and you run out of questions to ask, making learning more in that domain more challenging.
How can we encourage people to continue to ask questions as their expertise grows?