The game Zendo is an icehouse game of inductive logic. There are many different variables possible in the game (size, shape, orientation, pointing, etc...). One way of limiting rule difficulty is to limit the number of variables allowed in the rule.
What is your favourite set of variables when playing with new players? What's the best way to introduce new variables to escalate difficulty? What variables have caused more problems then they're worth?
The most basic, and easily visible, attributes are best for beginning players are the most obvious and easy to see. Color, size, basic orientation, number of pieces.
Rules that involve simple combinations of these attributes are good (a koan has the Buddha nature if it has a small red piece). Rules that have too many relations going on (touching, pointing) at once are somewhat more difficult (a koan has the Buddha nature if it has a small red piece touching a large green piece). Rules that involve pip count arithmetic can be quite difficult for beginning players. Rules with an "and" in them (a koan has the Buddha nature if it has a red piece and a flat piece) are OK for beginning players, as long as they aren't too complicated; an "or" (a koan has the Buddha nature if it has a red piece or a flat piece) are more difficult and should probably be avoided at first.
There are a few features that can be fairly non-obvious or hard to express for beginners (groundedness, weird orientation) and so should probably be avoided at first and introduced slowly. Also, introducing non-standard relationships (corner pointing, nesting without touching) should be avoided until people have a little more familiarity with the game.
Another way to simplify the game for beginners is to remove one color. With only three colors, it becomes much easier to figure out and test rules that involve colors.
One thing to remember with Zendo is that it's almost always better to err on the side of a rule that's too easy rather than a rule that's too hard. It's frustrating when no one figures out the rule in Zendo. It's very easy to come up with a rule that's too hard, especially if you're trying to be "clever" about it (one of the more frustrating rules I've played with was "the sum of yellow and blue pips must be a fibonacci number"; that was especially bad, because 0, 1, 2, 3, and 5 are all fibonacci numbers, and you run out of pieces quickly when trying to test higher numbers to figure out the pattern).
If all the players are new, then I think the game includes some sample rules on cards. That doesn't work if some of the players remember the sample rules.
My rule of thumb for creating rules in this kind of game is to either combine two dead simple rules (No blue pieces and more than three pieces), or to have one slightly sophisticated rule that always applies (the top of each piece must be at the same height or above the tops of all pieces with fewer pips).
Of course new players have to be reminded that their rules are always harder than they think they are.
