Nice suggestions all around!
My suggestions:
* Weapons made by inferior materials use the stats of standard weapons. Metal weapons give bonuses. Otherwise a stone spear will be a worse choice of weapon than a wooden club, something that doesn't feel right. An iron weapon is +1, other materials give their usual bonus +1.
* When PHB II comes out with psionics, all starting PCs get one level 1 psionic power in addition to their regular powers to simulate wild talents.
* If there are no rules for light-armoured defenders in the RAW, defenders get +2 AC level 1. This is to compensate for the lack of heavy armours, that bonus plus Dex/Int- bonus puts an Athasian fighter or paladin at even ground with metal-wearing defenders in other settings.
* By adding a touch of personal cult to the power of their elemental vortexes, the sorcerer kings grant divine power in a different fashion than the power the clerics gets. Therefore, templars use the paladin class.
* Druids make pacts with the spirits of lands and other entities that awake due to the massive damage that is caused to Athas. They use the fey- pacted warlock class. The Dragon of Tyr puts a massive impression on the arcane flows of Athas. By utilizing the arcane movements of terror and agony, some people gains dark powers. They use the infernal warlock (fluff- wise, they are Dragon pacted warlocks).
Athas was thrown far, far away on the Astral Sea, losing connection with the other planes. So far out that some realms best not associated with have come close. Sensitive people feel their influence, gaining power. They use the Far realms- pacted warlocks.
* Clerics need re- fluffing, otherwise they can stay the same. Ambitious DMs may put different powers under the spheres of fire, water, earth, air or cosmos.
* As for races, elf, half elf, human, dwarf and halfling use the RAW. Dragonborn replace mûls. If you look in a Dark Sun- sourcebook, you see all those people with strange looks; they are perfect for tieflings. It is a different breed of humans, being created partly out of some twisted evolution, partly due to the magic that ravashes the world. They are cunning, brutal and possess strong personalities. They are refered to as "freaks" when someone feels like distinguishing them but otherwise they mix fairly well with humanity.
* If there are PC- rules for some big, strong and stupid race in the MM, that can be used for half giants.