I appreciated having athas.org around last month when I ran a Dark Sun one-shot. I didn't necessarily use the rules exactly as provided, but it did give me a lot of ideas and resources to use.
I have to say I'm still looking for the ideal defiling rules. What do I mean? I need the rules to support the following:
* defiling is more powerful and/or easier than preserving -- making the choice not to defile should bear a cost of some kind
* a preserver could, at a moment's notice, start the slide to the dark side by defiling
* defiling is more attractive, in some way, than psionics (why would you bother with arcane magic if psionics are just as powerful?)
* defiling aids the caster and somehow hinders those around him, particularly water-related and/or plant-related creatures
* defiling somehow becomes addictive -- it's hard to redeem yourself after you've gone down that road -- and when you're a defiler there's little choice but to defile when you can
* defiling has more effect in verdant areas and less effect in lifeless areas
* a way of keeping track of what areas have been defiled
* probably some other stuff I'm forgetting