Dan,
The definition of Quirk is "Something that is strange and unexpected; A sudden twist, turn , or curve."
Most often Quirk is manufactured and that is why I say it should never be sought. In other words, don't create something just to be quirky. That doesn't mean that an architect shouldn't create something that may be different; just don't call it a Quirk. Because it isn't.
To me, a quirk is something that results from a site constraint that just can't be changed. This is different than something that is just thought-provoking, cool-looking, or just simply different. To illustrate, I use the 3rd hole at White Bear as an example of a quirk. Such a short and small par-three, the hole is a result of the routing of the golf course coming up against the property line, which has a high slope.
The hole is a result of its site-constraint as opposed to the same exact hole that may appear somewhere else in a routing solely by the architect's choice. That hole, in its same exact form, designed somewhere else without any site constraints, is equally thought-provoking, cool, and certainly different. It just isn't quirky in that location.
One of the primary goals with this book is to truly drill down to the actual definitions of the principles that we all take for granted and tend to generalize about. As a result, I try to make the distinction between Randomness vs, Chance; Fairness vs. Playability; Difficulty vs. Penalty vs. Challenge, etc.