From the descriptions I've read over the years, and the profiles here by Sean and others, I've concluded that with so many of these GB&I courses, "fun" is not the opposite of "hard/difficult".
Many of these courses are not "easy" or "easy to score well on", but they are "fun" nonetheless because they tend to be neither "long" nor "predictable/straightforward", nor even obviously and carefully "designed".
I know that's an obvious point and a common observation. But I find it interesting because it goes back to our many "hand of man" and "found" vs "built" discussions.
I wonder if our sense of "fun" has more to do with the feeling of youthful "discovery" (as Sean notes in the link Bernie posted) than it does with specific architectural philosophies/approaches and values, e.g. width, green contours, playable rough etc.
Some astute music critic once described improvised jazz as "the sound of surprise". So many of these courses seem to feature "surprises" around every corner, and genuine surprises too, i.e. not the kind that an architect who has planned every hole and moved a lot of earth to create those holes could possibly duplicate, simply because the very act of "planning" precludes the kind of randomness that Nature provides.