Though the demo was on the short and easy side, the game shows enormous promise.
I've never played Picross before in my life, but after playing a bit of Picross 3D at the show, it seems that it's about time to rectify that. The Picross 3D demo was a fairly short affair, consisting of about six levels and a few short tutorials explaining how to play.
Whether or not the rules of regular Picross are the same, but the rule set of Picross 3D was fairly simple. Each puzzle started with a collection of cubes organized into a larger cube. On the edges of this larger cube, there were numbers written. These numbers signified how many smaller cubes should exist within that row at the end of the puzzle. In other words, if one row had a zero on it, that meant you could destroy all of the cubes in that row without hesitation. Destroying a block was one of the two possible functions you had at your disposal. You were also able to mark blocks so you knew not to destroy them. Typically from cues on other rows you would be able to make assumptions of which blocks you shouldn't destroy.
As mentioned earlier, there were six levels total in the demo. The second set of three levels were actually a part of a tougher tutorial with more advanced rules. The tutorial explained to me that numbers at the end of a row without a circle around it meant that the remaining cubes in that row were all consecutive. Obviously those with a circle meant non-consecutive, which allowed you to take other leaps of faith when destroying or marking other blocks. This new rule gave further complexity to the puzzles.
While I may not have made this clear, the purpose of Picross 3D is to clear away blocks in order to reveal a hidden object within the large cube. The cues given are basically a road map to revealing that shape, and it is up to the player to determine each particular direction.
Picross 3D was enormously fun and I'm greatly looking forward to revisiting it when it releases.