I have been hard pressed to define quirk. I think we have had threads in the past, attempting to get a handle on quirk.
It is sort of like the Supreme Court Justice stated about porno, "I know it when I see it".
Quirk most often is found on older courses. It is that oddity that just didn't fit quite with the rest of the theme, or presentation that the architect was offering, usually due to a unique feature of the land, that caused a brainstorm for the designer to think outside the box and work around or inspite of a particular problem. It may be an isolated terrain feature, and obstacle, or a space problem, or a need to do something odd to cope with a soil or drainage problem. In any case, it caused a departure from conventional wisdom and so constructing a golf hole still presents the player with the oportunity to also depart from conventional wisdom if they choose, and still play the hole within the context of the game. Quirk adds an element of discovery and excitement to deal with something different.
With 20,000 or whatever golf courses, and most of them following some basic principles of conventional wisdom in construction techniques and design themes, it becomes a test to even distinguish one designer from the next. But, when we encounter quirk that is not outrageous and beyond the bounds of the game where a designer coped with it in a clever manner and when that quirk makes us think and amuses, then it will set the course that it is found on, apart. That bodes well for that particular course's recognition in the ratings game, I think.