The Great Openness is a term used in the Zhuangzi only in passing, yet it is one which I find most evocative of the Daoist vision.
When I speak of the Daoist vision, I mean that hypothetical, psychological experience of being in the world which is uniquely Daoist. I approach it as merely hypothetical not only because I have not experienced it, but because I suspect the one who originally used it did not experience either. In that case, this and every comparable reference to the ultimate experience of sagacity is only an ideal to which to aspire and is realized only by approximation.
I understand it as psychological because, in the absence of any real knowledge about Reality, every experience of It is necessarily subjective. It does not inform me; I inform it for my own purposes. And these are intended to create a psychological experience most conducive to tranquility. As I so often say, no Truth is on offer here. I simply don't know any. Thus, I can only suggest, Do you wish to experience tranquility? Try this. Or don't.
The Great Openness, like every expression of sagacity, is Dao-likeness. Dao impartially embraces everything without discriminating the 'good' from the 'bad', the Dao-like from the non-Dao-like. Yet Dao cannot truly be said to be Open, since this implies the possibility of Closed. In this sense, the Great Openness as a concept is predicated on human psychological experience. It is a 'place' which is only a state of mind.
In a previous post I said that openness is not best likened to opening a door into oneself, but rather to turning oneself inside-out. The Great Openness is not a selective openness on a continuum with closedness, but an absence of selectivity. What this implies is a loss of one's insular identity, where identity assumes intrinsic difference. And this implies the loss, among other things, of identification with opinion, on the one hand, and the acceptance of every identification with opinion, on the other. One need not affirm the content of an opinion to affirm the one that holds it. Zhuangzi, in his "equalization of the assessment of things", did not say that every opinion is 'right', but that every opinion is both 'right' and 'wrong'; in this, they are equal.
The Great Openness is an experience of being which is best imagined by the individual. In other words, I have digressed into my own imagined meanings. Since it is presented by Zhuangzi, however, it might be helpful to remember other ways he spun it: It is "far and unfettered wandering" in "the vast wilds of open nowhere", "our homeland of not anything at all."
You can check out Scott's writings on Zhuangzi here.