One of the more interesting versions that has seen it's fair share of discussion is the concept of insanity and knowledge of the Cthulhu mythos in the game Call of Cthulhu. Basically everything in this games works of a percentile system. There are a few ways that a character can be removed from play, but these predominantly fall into the categories of "Death and Injury", or "Going Insane".
Characters start with hit points that measure their physical injury, these can be rapidly depleted through conflict, but they can be healed. Sanity is a slow loss, but it can't be healed (except in a few variants in specific sourcebooks and home rules).
One of the major destroyers of sanity is knowledge of the dark, macabre and malevolent spirits that exists on the edge of reality. Once you know that humanity is essentially damned and insignificant compared to these powerful creatures, you lose all hope...
You start with a cap of 100% in your sanity score, but this cap is reduced by an amount equal to the character's knowledge score in the cthulhu mythos. 60% Cthulhu Mythos means a maximum of 40% sanity.
I've only ever played a couple of games of Call of Cthulhu, but I have been a long-time fan of the works of HP Lovecraft. The few games I've played, have come closer to embracing the spirit of the stories and novels of the mythos, so I guess this system is successful in this regard.
A flip side to this relation mechanic is something that you find in many of the White Wolf games, for example the generation/blood potency mechanism which is used to control the maximum level of discipline powers which can be obtained by a vampire.
In this sort of system you have to improve one value before another can be improved.
Both of these have merit, but I'm actually more interested in the two types of value that impede one another. These mechanisms force players to make choices about their characters rather than simply providing methods to channel experience point expenditure along specific lines.
I'll follow the previous line of thought with white wolf's Vampire line; a few home-brew impedance variants I've enocuntered for Vampire games relate to the humanity mechanism.
In one variant, the maximum degree of power that may be learnt by a character (on a scale of 1 to 10) is restricted by the character's humanity score (also on a scale of 1 to 10). This is applied on top of the existing mechanisms; meaning that characters must decide whether to degenerate into inhumanity if they wish to claim the most powerful levels of mystical mastery, or they can maintain their control and humanity in exchange for only ever learning the smallest levels of mystical chicanery.
Strangely, this fits better with the World of Darkness mythology than the basic game seems to... so it's hardly surprising that I've seen variants like this implemented.
Other variants get a bit more complicated, by using formulae incorporating "blood potency"/generation, humanity scales and possibly even ther factors to determine the maximum potency of the mysteries that may be learned. But once you start getting complicated, you move away from the things that I find interesting, the moral choices.
When it gets too complicated to calculate quickly, it get's too complicated to reflect the gut instincts and subconscious thought patterns of a character. Conscious thought precludes subconscious thought.
Another interesting variant on the inter-related values idea can be found in the Palladium game Nightbane. (I figure that I had better include a positive critique on a Palladium game, after all the bashing I gave them last week).
The inter-relation here is that characters have a maximum energy capacity to fuel their powers. They can either learn a basic number of abilities and have their full fuel capacity to use their powers quite often. Or they can sacrifice their total fuel capacity in exchange for the ability to learn more powers.
Do you become a one trick pony able to use your power quite often? Do you have a diversity of powers available which can be used only every now and then? Or do you try to balance between these extremes.
It makes players think, and that's the whole point of roleplaying isn't it?