I'm A Banana
Potassium-Rich
But if defiling is a "dirty trade secret", an "add on" to the normal routine of casting, which you chose to add strength to the effect by defiling life energy form around you...then everything works out neatly
Totally cool with that.

I do think defiling should be an option for players. The allure and appeal of the "dark side" should be there. You don't have to use it...but it's always there, saying, "hey, wouldn't it be nice to kill these guys a little faster? and will the world really notice just one more little patch of grass?"
Most of the classes shouldn't be a tough fit once you determine what the power sources are drawing on. If all "divine" classes are in service to a sorcerer-king, and all "primal" classes are in service to the elements, and all "arcane" classes have the preserving/defiling option, the individual classes more or less run together.
The thing is, the disconnect from the planes is pretty important to me for the feel of the game. There can be no "fiendish blood," and any "outsider pacts" (even with elemental princes) would leave a pretty bad taste in my mouth. The idea of Athas as an isolated world where there are no angels or demons or outsiders, only forces, is key to the brutality of it. There's nothing to relate to, no mirror you can hold up, nothing alongside this world. There is this world -- this blasted hellscape -- and nothing else. Even death is just nothingness. Everything has to live under the same environmental devestation. The inter-connectedness of the ecosystem is kind of kicked in the junk if the ecosystem is made of magic like the Feywild. Defiling has broken nature; the desert environment isn't a natural desert, and seeing intact places of natural wildness is just dissonant (let alone the fact that mighty wizards like the Eladrin live there).
Most of that is just a re-fluffing issue, though some of it is more difficult (teleportation, in DS, has to be explained as something other than "going to a nearby plane for a while"; any race that is naturally inclined to be a wizard, bard, or other arcane class, is a good target for the Fey Genocide).
If, like Bill said, they're not going to shoehorn 4e aspects into DS, dropping the Feywild and ruling that all fey live on Athas itself goes a long way for it. Mentioning Eladrin and Gnomes only in passing as "races that have been killed" also goes a long way for it (and yes, you can still play one of the unique "survivors of the genocide," but the setting will not assume you can play it).