The Dragons Tyr
There some big changes to the classes in Dark Sun:
The Divine Power Source
The Divine Power Source does not exist on Athas as the Gods deserted it long ago. Priests and Paladins are non-existant.
The Arcane Power Source
The arcane power source is tainted. When users of Arcane magic cast spells they naturally draw energy from the plants and animals around them. The ground becomes barren and those close to them feel intense pain as their energy is sapped. With care and concentration they can learn to control this and preserve the land around them however even a caster who protects the land when they cast will still not be able to avoid the stigma attached to magic users.
The Psionic Power Source
Possibly due to the absence of the Divine or the prevallent use of the Arcane most people on Athas posses some talent for the use of Psionics (commonly referred to as the Way). This can range from increasing the strength of hearing, seeing from a viewpoint some distance away or being able to create simple tools from thin air.