Since wabi-sabi represents a comprehensive Japanese world view or aesthetic system, it is difficult to explain precisely in western terms. According to Leonard Koren, in his book Wabi-Sabi: For Artists, Designers, Poets & Philosophers, wabi-sabi is the most conspicuous and characteristic feature of what we think of as traditional Japanese beauty and it "occupies roughly the same position in the Japanese pantheon of aesthetic values as do the Greek ideals of beauty and perfection in the West."
- "Wabi-sabi is a beauty of things imperfect, impermanent, and incomplete.
- It is the beauty of things modest and humble.
- It is the beauty of things unconventional."
- (quote from Koren's book)
- It is the beauty of things modest and humble.
The concepts of wabi-sabi correlate with the concepts of Zen Buddhism, as the first Japanese involved with wabi-sabi were tea masters, priests, and monks who practiced Zen. Zen Buddhism was first brought to Japan from China at the end of the 12th century. Zen emphasizes "direct, intuitive insight into transcendental truth beyond all intellectual conception." At the core of wabi-sabi is the importance of transcending ways of looking and thinking about things/existence.
- All things are impermanent
- All things are imperfect
- All things are incomplete
Material characteristics of wabi-sabi (again from Koren's book):
- suggestion of natural process
Wabi-sabi appears in following aspects of go:
- natural stone placement
- stones won't fit when lined up 
- wood grain, colour palette, and natural degrading of a goban (see also kaya)
- structure in stones
- non-rectangular dimensions of the goban
Bonobo: A few thoughts:
I think the last point should be “non-quadratic”: the board is stretched between the players in order to eliminate perspective distortion that would make it appear vertically shorter than horizontally. Only valid for Real Life physical boards.
Also, perhaps it is less about “imperfection” but rather about appreciating difference and asymmetry? Like …
- the black stones are slightly larger than the white stones
- the white stones are glossy while the black ones are matte
BTW this also reminds me of Kintsugi, “the Japanese art of repairing broken pottery with lacquer dusted or mixed with powdered gold, silver, or platinum” (copied from https://en.wikipedia.org/wiki/Kintsugi). Could perhaps also be applied to Go, in that we sometimes win a game that we had already lost?
It is also two separate words, with related but different meanings.
Wabi is the kind of perfect beauty that is seemingly-paradoxically caused by just the right kind of imperfection, such as an asymmetry in a ceramic bowl which reflects the handmade craftsmanship, as opposed to another bowl which is perfect, but soul-less and machine-made.
Sabi is the kind of beauty that can come only with age, such as the patina on a very old bronze statue.
For more about wabi-sabi, see
Fine quotes relating to both words separately . . .
 Although I haven't seen this in reality
unkx80: Do you consider the case where the stones intended for a larger sized board is played on a slightly smaller board? During the endgame stage, placing a stone on the board in this condition is a very tight fit - like a child who has outgrown his or her clothes.
Aselan?: I actually have an goban where this issue came up. We had a discussion about the fact that "stones won't fit when lined up". And I also posted pictures: https://www.boardgamegeek.com/thread/1572277/slate-stones-doesnt-fit-board
Meanwhile I could compare it to a board where the stones fit. And I must admit that I like the look of "stones won't fit when lined up" far better. So I'm a Wabi Sabi Fan now. I had some issues to adopt to it, but it's far more relaxing to play that way since you simply can't set the stones perfectly anyway, so you don't have to try. Since I'm a perfectionist I had a real struggle with it, specially since I payed a lot to get "the perfect stones". Each time I play with my board I'm reminded to Wabi Sabi and that nothing is perfect in this world and trying to make things perfect is either too much effort or simply not possible. This makes my games on this board more than just a game of go :-)
Also the stones have to slip above and under each other when the board becomes full of stones. That causes a more "organic" and "dynamic" look of the game situation. The funny thing is that the white stones are lower what usually causes the black ones to become inclined. This makes the black stones look a bit humble. And since the weaker player usually plays black, it may (in some sense) represent the more humble play of the weaker player. Which I find nice...
PJTraill: Perhaps not so unfamiliar in the West, as witness Herrick’s Delight in Disorder:
A SWEET disorder in the dress
Kindles in clothes a wantonness:—
A lawn about the shoulders thrown
Into a fine distractión,—
An erring lace, which here and there
Enthrals the crimson stomacher,—
A cuff neglectful, and thereby
Ribbands to flow confusedly,—
A winning wave, deserving note,
In the tempestuous petticoat,—
A careless shoe-string, in whose tie
I see a wild civility,—
Do more bewitch me, than when art
Is too precise in every part.
— though maybe this is more coquettish than wabi-sabi.