The tremendous stress and constant conflict between people is why people are higher level on Athas than they are in other worlds.
Defiling makes sense as a way to destroy the world as not only does Athas not have an equivalent to the Weave, Siberys, or Gods to provide them magic but Raajat seems to have planned to utterly rebuild the world from the ground up.
The Gith deciding to go Screw This, I'm Outta Here makes sense with the fact that the Sorcerer Kings are Epic Mage/Psioncists that could actually slay their queen, Vlaakith. In the Astral Plane, they would be utterly invincible and capable of enslaving the entire Gith race that has no members above 13th level.
There might well be many other places with life outside the Tablelands but given its preponderance of Evil OverlordSorcerous Overlord types, you can understand why no sane person would try to travel to it.
Mul being a name for half-dwarves is obviously a Fantastic Slur when you realize they're sterile products of crossbreeding. It is sadly also a slur in the real world.
The Feywild being effected by defiling implies that the magic is actually reaching across dimensions. Who knows what sort of damage the magic is doing to the Multiverse.
How did defiling magic destroy the oceans of Athas? If most of the planet is a desert then any recovery has to be purely magical.
