Alot of my thoughts on how they'll impliment various classes and mechanics are already covered, but here's the overall philosophy I would take if I were them:
Don't tinker with individual classes unless absolutely necessary. Instead, tinker with the power sources.
Eg. Preserving/Defiling should be represented by things like feats and paragon paths. That way everybody from sorcerors to swordmages to any random arcane class they come up with in the future can be either a defiler or preserver.
Divine classes as Templar variants. The cleric matches most closely with a Dark Sun templar, but I could easily see the Sorceror Kings imbuing people with divine energy to serve as bodyguards(paladins) or assassins(avengers), or for many other purposes.
The former 'elemental cleric' role is taken over by the primal power source classes. Why would an independant elemental worshiper be called a cleric anyways? Never made much sense to me. Shaman seems to be a much better fit.
The other big key is avoiding getting entangled with old school names. Cleric is a good example of that. Why bother totally remaking the class that happens to be named 'cleric' into the old school elemental cleric when the shaman is far closer to it already and the cleric class can be reflavored into templar, which it matches pretty closely? The fighter is another good example. Dark Sun fighters were leaders of armies, generals, and the like. We've already got a class that does that: the Warlord. Additionally, the fighter fills the Dark Sun gladiator role pretty well, so why bother making a gladiator class? Just give the fighter some gladiator flavored stuff. 4e rogues can easily fill the Dark Sun bard role, so we shouldn't see the 4e bard class rebuilt to look like the Dark Sun bard either.
A ton of good points as to how to think about the new setting.