So I doubt I can say much. But (I love to talk) I will take a shot at the aspect I find most interesting, which is the game's presentation of its point of view. Your point of view? Both.
So when I heard that Jon Blow's next game would be puzzles on a mysterious island, I said "Oh, he's doing Myst now." Myst is as much a videogame archetype as Adventure and Tetris. Taking apart Myst's conventions and assumptions won't necessarily make a great game (it might get you no farther than Pyst did) but it could be an excellent launching point.
You are you; the game is your view of the world; you act by manipulating the world directly. These ideas were never perfectly implemented -- the original mouse cursor and 544-pixel-wide window strained to hold the illusion of being your hand and your eye. But the ideal seemed so obvious as to require no argument.
(* Until near the end. We'll get there.)
You are you. The first-person view of Myst, like the second-person prose of Adventure, projects the world around a blank space which you invisibly inhabit. Your character has no voice, no body; your hand is abstracted down to a cursor.
Many adventures after Myst (and several before it) tossed this faceless ideal away with great force. Strong characterization serves most stories better than the invisible avatar -- what a later adventure mocked as the AFGNCAAP. And, of course, the blank protagonist isn't all that universal to begin with, not as long as "unmarked" still means "white, male, straight, not too old, not too fat..." (Yes, I've used the faceless protagonist in my own games. But I don't pretend that it counts as representation.)
The Witness lets you inhabit that blank space -- at first. You have a few moments to settle in and imagine yourself walking around. Then you emerge into the sunlight, and... perhaps you still don't notice your shadow. But when you do, the shadow is tall, lanky, short-haired, trousered, male. It's definitely not me. Is it Jon Blow? I certainly can't think of any other candidate, so let's assume that you play The Witness as a mute Jon Blow.
But why? "You" have no voice or background; the game does no work of characterization. But neither does it allow you to fill in your own. You are left a liminal, uncertain presence.
The game is your view of the world. Again, you initially have no reason to doubt this. The game's art style is not hyper-realistic, but we're all accustomed to visually stylized environments by now. Perhaps it's unusually low-poly for a modern game ("ironically low-poly", one friend commented). But then a lot of subtle work went into the texturing.
I figure the style was balanced to allow panoramic views across large swathes of the island. The Witness is generous with those. (Contrast Firewatch, which mostly hems you in with ridges, canyons, trees, and foliage to avoid the rendering cost of the whole world at once.)
Then you discover one of the game's more subtle puzzles, those of visual perspective. Why do two sticks, a rock, and a distant fence form that shape? It represents nothing in the world, but the game wants you to take notice.
Should we take the world as a purely visual contrivance, then, lacking physical reality? The perspective puzzles incline us that way, but then the island does have a physicality to it. Some clue-objects are bent or broken, implying a physical history: this twig snapped off and fell. A cable used to connect over there. That post was straight until someone leaned on it.
Again, we are left uncertain. The world wants us to believe every leaf was laid just so, but also that it is subject to physical decay. Why?
You act by manipulating the world directly. The first interaction most players encounter in the Age of Myst is a knife switch; you grab it with your cursor-hand and pull it down. From there, the game extends the arms'-reach metaphor in subtle but definite ways: you press buttons, pull chains, hold a lit match. (Plus, of course, the initiating moment: laying your palm on a magical book.)
Your first interaction in The Witness is a panel with a line on it. You drag the cursor along the line to activate it. For adventure gamers, the implication is clear: you reached out and swiped your finger along a touch-panel. (If you are my age, you went "Dzzzzzhhht!" like Kermit the Frog drawing a letter.) And you go on for quite a while, finding panels and tracing lines on them with your finger.
Only, maybe not. You might notice that your shadow, the ambiguous Jon-Blow-or-not, never reaches out to touch anything. According to your shadow, you're just standing motionless in front of each panel. A lazy animator, not bothering to construct the arm-motion? But you can see your shadow-feet shift and your shadow-head turn as you look around. This game does not scant the details.
It soon becomes clear that The Witness consists entirely of these path-tracing interactions. There is not a single lever, dial, or key in the game. Furthermore, you don't have to be in arms' reach to trace a path! The game makes it convenient to stand directly in front of each panel, but you can activate any path you can see. It works from any distance, as long the entire path is visible. (The visual-perspective puzzles hammer this point home, if you overlooked it.)
So we must give up the idea of swiping a finger along a surface. Touch has nothing to do with it. You never manipulate the physical world (if there is one!) in any way(*). Indeed, if you look closer, the island is most unwilling to react to your physical presence. You can hear your footsteps, but you leave no footprints, nor even ripple the surface of a puddle you step in. You cannot brush aside a twig or pick up a bit of paper to read.
You are a ghost, or a shadow of a ghost. Do you interact by observation? Perhaps you are simply recognizing the paths, and the panels react to that recognition. Or perhaps you are playing a game, manipulating it with your mouse or controller? Perhaps there is no metaphor at all.
But if you're a ghost, you're a ghost with eyelids and retinas. (Someone had to point this out to me! Hit the pause button; watch the solar afterimages fade.) And we like immersive metaphors, anyhow.
(* In a couple of places, the game seems to implement "pressure plates" -- triggers that activate when you stand on them. This might be a physical interaction, or it might be reacting to your presence (or shadow!) in some other way; it's not made clear. I'll let it slip by.)
So The Witness leaves us off-balance, uncertain in our presumption of how adventure games work.
We might question whether "adventure game" is the right label at all. Is this island just a pretty picture with abstract puzzles pasted on? No, that description is inadequate. The physical laws of the island may not involve you, but they exist -- sunlight, shadow, reflection -- and you must apprehend them to solve the game. You must consider how buildings connect and how they might have decayed. Those are the understandings of an adventure game. And there are, after all, gates and drawbridges and elevators to play with -- even if you do so by the tracing of control-paths on panels.
These ontological musings do not slow you down, regardless. The puzzles are before you and you work your way through.
(They're brilliant puzzles, by the way. This post is not a review, but I didn't want to leave that out.)
...And then you reach the end-game (or the post-game, maybe, or the epilogue). I said up top that The Witness has "no explanation"? Play far enough and you get, mm, not an explanation, but an indirect trickle of clue. You can make some guesses. If you pass through the post-end-game (post-epilogue?), you find a cut scene which exposes a little more information.
I won't spoil it, except to say that the game's motif is perception -- clarity, perspective, focus. That's the title, right? It (kind of) makes sense that you, the witness, are (sort of) seeing out of Jon Blow's eyes, and your presence is sort of perceptual (but not exactly), and the island is sort of physical (but not really).
But a theory isn't a justification. We can still ask why Jon Blow (the real one) wanted to make a game called The Witness, in which you are a ghost with his shadow. Yes, it's all a thematic package, but why that package? You have to bear the uncertainty through most of that game, after all, before the half-explanations ever appear. If that shadow is discomfiting, you spend a lot of time uncomfortable.
It's not an enticing discomfort. It says: something is wrong here, but don't ask why. It invites you to withhold rather than speculate. This is subjective, I know, but I never reached a point of saying "Aha, now it all makes sense in retrospect!" At best it was "Well, I have a theory which could be thematically consistent with it all."
In the end, I must chalk it up to an aesthetic disagreement. Jon Blow wanted his adventure game to be distancing and not-quite-immersive. He chose a theme and style (and title) which suited that effect. It worked. It's not what I would have done, and that's all I can say about it.