For one thing, there's the changing-details-without-drawing-attention-to-the-change factor, namely the hyacinth/spider/cactus. It's a particularly interesting choice because the plant changes come amid a whole lot of other changes that you are aware of--so it adds an air of uncertainty to the confusion, to the extent that that makes sense. My initial impression, in fact, was that there were more changes going on below my sight line; as it happens, I was wrong, but the detail of the changing plant is enough to give the player that feeling (i.e., that not only are things acting funny, they're doing so behind your back as well as in front of your nose). The book also gives you slightly different readings at different times.
There's also that favorite of IF theorists (this one, anyway), the player-PC relationship, which takes a few odd turns here. For the first few items that crumble into sand, the player and the PC appear to be in the same position--something like "huh?" As the apartment disintegrates, there's an inevitable shift for the player--he/she sees the trend and says, okay, to get the story's conclusion I'm going to have to seek out more things to turn into sand, so bring it on. (I actually didn't realize that the list adapted when I first played the game, so I was reduced to wandering around the apartment poking and prodding things in hopes that they'd turn into sand, which obviously heightened the effect a bit.) What's interesting is that the PC's approach also changes to follow suit--the PC appears to welcome the disintegration after a while, find a certain perverse pleasure in watching everything crumble. To wit:
The toilet? Yes, it's the toilet's turn! You press the handle, grinning maniacally. And indeed, the sand rushes down the sides of the bowl. Carefully? You shove a paper-stack off the desk; it's a shower of sand before it hits the ground. Ha! You push another, and another, and then sweep the whole mass over the edge. White sand cascades everywhere. Laughing, you feel the desk itself give way. The cabinet and counter start to groan as soon as you touch them. You slam the cabinet for good measure; and the stained pressboard crackles white, shivers, and explodes into sand. All right!There's more than one way to look at this, of course; you could take it as a sign that the PC is beginning to lose it, or simply doesn't care. But it's also one of many meta-IF moments--where the game breaks the fourth wall, addresses the player directly, rather than observing the usual player/PC distinction. The computer is one example--"right now, however, there's a game on the screen -- one of the text adventures, or interactive fictions, or whatever they are this month"--but there are plenty of others. One of the things you hear on the radio is "Sharp words between the superpowers," which is the first line of Trinity. (Naturally, you comment to yourself "there are still superpowers?" An unremarkable line in 1986, somewhat out of place now.) There's also this: "The far side of the mirror is just as shadowy as this one. It's probably meant as commentary; not more space, just more of the same." "Shadowy" sounds like a So Far reference, but the "commentary" line yanks the player out of the story altogether by drawing attention to the author. There are also odd bits of humor here and there--if you read the list during the move when the kitchen ceiling is about to cave in, the list will say "stay out of kitchen!" At another point: "Another month of this and you'll indistinguishable from this apartment -- beige, featureless, and up for cheap rent. And the songs on the radio, of course, are mostly comic relief.
Myself, I didn't guess the game's author (though I didn't think about it much either), but I should have--partly because of the random song generator (reminiscent of the cave generator in Hunter), but even more so because of the taxi scene, in which the player opens the door and is suddenly overcome with a fear of what's outside. No explanation for the fear, no rationalization of why you'd prefer to stay inside, just--no. Try to open the door, once you've already opened it and looked outside, and you get "You do not want that." The parallels to Change in the Weather seem obvious, in retrospect--nameless horror that the PC won't deign to put his finger on, inexplicable dread of something you eventually end up confronting anyway. (Well, sort of, in this case--there's nothing there to confront other than your own consciousness, though that's plenty scary in its own way.) Shade is also classically Zarfian in its opacity--what exactly is going on when the apartment flashes back to its original state is anyone's guess, and while it 's easy to simply chalk it up to the general hallucination (which is fading at that point--you can no longer interact with the illusory scene), it's also tempting to try to read more into it. Ditto, of course, for the last scene with the tiny figure. Certain key points in Change in the Weather and Spider and Web were likewise open to multiple interpretations, and the author naturally has never resolved those ambiguities.
I don't really have any good guesses about the ambiguities, myself. My first guess was that the figure was your subconscious, and the "my turn again" meant that your mind is yielding to hallucination again--but why "you win", and why is the figure "dead"? Other ideas have been bruited about, most of them more plausible than that one. I do suspect, though, that Shade was written for effect, written to weird the player out, and that dissection and deconstruction on this level--symbolism and such--probably weren't the main things Zarf had in mind. It's possible that the significance of the figure lies somewhere between nothing (more hallucination), and mere weirdness and perversity (a way of injecting a memorable/chilling last line). The heart of the game for me, though, remains the moment when I opened the jar of peanut butter, since that was a turning point of sorts, when I stopped trying to impute rational explanations. It was one of the most unsettling IF experiences I can remember.
Had I realized that there was a less cumbersome way to move things along than prodding everything in the apartment, I might have considered giving Shade a 10, as it was, it got an enthusiastic 9 and a special place in my IF library.