I think you may have missed his point slightly. He isn't questioning whether one can identify important results after the fact. He is questioning whether one can identify important discoveries that haven't yet been made. In many fields of inquiry one can assemble a list of important open questions. The issue is whether if you keep that list and check back whether 1) any of those questions have been answered 2) whether the answer turned out to be important 3) whether other questions not on the list were answered and turned out to be more important. He is arguing that 1 and 3 are the more likely outcomes so that a question being on such a list is likely not a good indicator that it is a fruitful avenue of research. Some examples would be things like grand unified physics theories, the Riemann hypothesis, whether P=NP. Will any of those be solved in the next 10 years, unclear, will something not on the list but also important be solved, chances are highly likely.