My one great love in game rules is a self-balancing system - one which lets you push it if you are willing to pay the price, but it pushes you right back. My favorite example of this is the magic system my son Klaxon designed for The Book of Jalan.
The system was a free form one, with magic skills that could be combined, along with regular skills, into a "weave" of great complexity, but that wasn't the cool part.
The caster's power and skill were treated separately. Power was inborn, but skill was learned. You could be amazingly powerful as a kid, yet you would have no control. I remember a game where the caster was able to teleport six people with his Apport skill, yet he could only Apport them a meter. That actually came in handy in an escape once...
That's cool, but that's not the self-balancing part.
To power their magic, casters burned their own stats. Each skill was associated with a physical stat - Telekinesis with Strength, Aport with Agility, etc. The more power you put into your weave, the more points it cost. Repeated casting of the same spell wore you down fast. Also the system's equivalent of HP was derived from the character's physical stats, and lowering your stat lowered your ability to take punishment. Some games, a caster had to be carried away from a scene because he was exhausted and unable to walk. One caster almost drowned in a rain because he had wiped himself out to the point where he couldn't move.
Magic wasn't permanent. In order to make an item magic, casters had to lock one of their points into the item. Thus magic items were rare, as caster seldom wanted to lock their power away out of use, and did not survive their maker. The magic dissolved when the caster died or when the caster released his power froim the item.
That's a self balancing system.
Oh, the game - I talk about it in the past tense, even though it's still on sale, as it just never sold much. It had problems, mainly the fact that the layout was perhaps my worst ever, but that magic system was very cool.
Klaxon is graduating from BC this year, and going on to get his masters in game design.