The best argument for gnolls on Athas (apart from their 4E debut) is that Dark Sun lacks any “dog men” type monsters. I wrote some thoughts about Athasian gnolls previously, here are some more thoughts to set them apart from the standard D&D gnoll.
Gnolls are naturally resistant to psionic powers, including the mindbending type. All gnolls have the closed mind feat, and consequently are never psions and can never learn psionic powers. This is one of the reasons why gnolls make bad slaves (explained in my thoughts about gnolls), but also good guards.
Laziness is one of the chief characteristics of gnolls. If they are “in the wild”, gnolls will actively hunt, but if there is an option for a less strenuous role like taskmaster over slaves, they always take that option.
Gnolls are among the “civilized” of the New Races, meaning that they can get along with others in the villages, towns and cities of Athas. Part of the reason for this is that gnolls occupy a particular niche, and do not compete with the freeman craftsmen or professionals. Nor are there any gnoll noblemen in the Tyr region, or any gnoll owned merchant houses.
Gnoll bouncers are very common in low end drinking holes in every city in the Tyr region. High end taverns never employ gnolls.
Athasian gnolls are neutral aligned, with evil tendencies.