Pretty much it was like the AD&D wizard except the arcane sorcerers of the exotic east have to adopt a taboo--or else they'd lose their powers. Some examples
* Cannot eat meat. * Cannot have more treasure than the character can carry. * Must make a daily offering (food, flowers, incense) to one or more spirit powers. * Cannot bathe. * Cannot cut hair. * Cannot touch a dead body. * Cannot drink alcoholic beverages. * Cannot wear a specified color of cloth. * Cannot light a fire. * Cannot sit facing a specified compass direction.
I remember in my wayward youth, being a good little munchkin, I was trying to make a Wu Jen with a taboo that wouldn't get in my way. And I couldn't. Can't eat meat? What if we're starving in a jungle? The treasure thing? Forget about it. Daily offering? What if I'm paralyzed for a day?
In other words, although most of these taboos have nothing to do directly with the mechanics of killing things and taking their stuff, any one you could imagine might get in the way.
Because in an RPG, any characteristic of your PC could possibly become an issue. That's how RPGs work.
Or, to put it another way, although wargamers divide rules into "crunch" (numbers) and "fluff" (background info), there's no fluff in pen-and-paper RPGs. It all matters, or could. ________
What I really like about Wu Jen taboos is they're one of the surprisingly rare examples of true "fairy-tale logic" in D&D. By that I don't mean it's just unscientific or magical, what I mean is the rule has the aesthetic qualities specific to fairy tales. I use this "logic" a lot.
The most prominent characteristic of fairy-tale logic for DMs interested in promoting it is that it deals in absolutes, not percentage chances or marginal cases. Halflings having better saves than humans isn't fairy tale logic--elves being immune to sleep spells isn't quite fairy tale logic but it's close, elves never sleeping unless they want to is fairy tale logic.
In fairy tales, effects follow causes with the inevitability of contracts between gods. There are no half measures. This makes the mechanical manifestation of fairy-tale logic nice and clean. Fairy tale monster powers should have no save--though they could have loopholes (vampires are the classic example: except at night, unless you invite them in, etc. etc.).
The long-term game advantage of this is that rather than sending PCs level-grinding around the setting looking for magic items with buffs, it forces them to create scenarios to exploit the logic.
(Interesting sidenote: fairy tale logic is the opposite of science but it is very close to how computers "think". Either/or, if this then not that, etc. It is easier to create a video game that works on fairy tale logic--"pac man can only eat a ghost after having a power peller"--than one that simulates reality.)
Like for example one of my least favorite things is the demon defense hierarchy: minor demons require a +1 or better magic weapon to hit, middle ones require +2 or better to hit, bigger ones +3 or better to hit, blah blah. That just ensures that the PCs have to have a mood-crushingly big market in magic items if they ever start fighting demons. Now if the demon can only be hit by a recently-despoiled virgin that has kissed a king during a war--well then there's like 9 adventure hooks right there.