A brief review of Myst forums reveals that I was wrong about a lot of things in my previous review of the demo. The Slate is used during the demo. Details that I had assumed were extraneous, such as the D'ni numbers and the symbols they correlated to, turn out to be vitally important. In fact, as it turns out I failed to make all but the most basic connections when I played the demo.
And yet, I managed to solve the puzzle and get to the demo's end, simply by using a brute force approach. There are four revolving pedestals with eight symbols on each one, each affecting a different door. It's not a combination lock, so it takes less than 32 attempts to find the right symbols. Which right there forces me to reevaluate the entire puzzle. You're not supposed to be able to crack a Myst puzzle by trying all the possible combinations, and one of the most important things a puzzle designer has to keep in mind is that as long as the number of combinations isn't prohibitive (and from my own personal experience, 'prohibitive' means a hell of a lot more than 32), most players will prefer the brute force approach to stopping and thinking.
As it stand, the puzzle also brings up some unfortunate questions from the previous Myst games, which I had hoped the End of Ages designers would have enough sense to keep in mind: why does the last man on the planet need combination locks? And even if we accept the combination locks, why would he leave himself hints to the combination which other people would be able decipher with ease? In Myst and Riven, the combinations to most locks were written in diaries, so that the challenge became getting your hands on the diaries, or, if the hints to the combinations were in plain sight, the locks themselves were hard to reach--in other words, a fairly realistic approach to security, implying the existence of real people who didn't want strangers in their secret hideaways. In Exile and Revelation, the locks were clearly meant to be openable by anyone with enough free time who happened across them. The game-ishness of the world began to peek through, destroying the player's suspension of disbelief.
Judging from the demo, it seems that End of Ages is taking the same approach, which is a shame. The quality I always liked best about the Myst puzzles was their integration into the game world. At their best, the puzzles weren't really puzzles but a series of hurdles that the player had to overcome, as if they had really been dropped into an unfamiliar place and had to figure it out as they tried to explore it. It's part of what made Myst so immersive, and it's saddening to think that the final installment in the series might not offer that same experience.