Working out whether a question is actually worth highlighting is a lengthy, deliberate, and subjective process, often informed by historical experience on what good mathematics was generated (or not generated) while working on earlier problems of this type. In particular, being aware of the “difficulty landscape” in a field—what questions are very easy to answer with known methods, which ones can be solved but only with some effort, and which ones are impossible—is of crucial importance in making such determinations.
[…]
[I]t is now the identification of a promising problem which is the scarce and precious resource.