I like some of the responses so far, but I agree most with Jeffo. Anyone can catch fish when they are biting. A really good fisherman can be measured by their consistency and ability to catch fish when no-one else can. Even better if they can do that with a variety of different fish, not just one species. I think a good fisherman should also be able to catch fish in unfamiliar territory, although it is hard to beat local knowledge.
Jeremy