I took about 10 minutes earlier to write up a "Highlight Pitch".
1) Regional Cultures. While there are different player races they're all culturally mixed up within the City-States around the area. Some tribal groups maintain "Racial Traditional" cultures, but mostly it's a blend.
2) Horrible Gods. A departure from Dark Sun that hearkens to older works. There are divine entities but they're largely either wicked or uncaring. There's probably a good deity but their worship is cult-like and small in the world while the other gods get constant appeasement to avoid harm.
3) Largely desolate environment. What arable land exists is largely owned by evil people and worked by slaves to provide food for society, most of which goes to the fewest people. Herding scrub-eaters and hunter/gathering are the most common ways for tribes to survive in the wild.
4) Small forests, sun-parched savannahs, lots of desert, lots of rocky terrain, mud flats and oases. Fragile ecosystems, everywhere, ready to collapse if abused.
5) Psionics are something that mark someone as "Different" or "Special". It won't get you a free pass to the king's table, but being born with Psionic Potential basically tells everyone you're probably going to survive to adulthood and be someone important. Also not something so trivially common that everyone has some.
6) Sorcery/Magic is largely viewed as FOUL because of terrible kings and horrible people performing profane rituals. Rather than defiling the world, it corrupts the body. Purification rituals have to be performed often to keep the naturally corruptive nature of magic from taking hold of the spirit. But people who purify themselves face no problems in the long term. (Might be short-term use problems, will work on a system later)
7) Ancient Ruins from Earlier Ages. Pretty much everyone "knows" that the gods were angered by mortals and sent down terrible curses and unleashed the yawning maw of hell. What great offense that was is lost to time, but might be found within the ruins.
8) Unknown cosmology. There is no Planar Travel magic. Maybe because the planet is cut off from the planes, maybe because the people just never strove to leave their world. There's practically no knowledge of the planes whatsoever beyond broad sweeping generalizations of the Gods living in their "Own" places and monsters coming from "An Evil Place".
9) Dangerous World. Between strange beasts, hell's spawn, harsh weather, and the curse of the gods, the lands are a place few can safely travel alone. Tribes can move through the wastelands by staying close together and in large enough number to ward of large threats, with enough cooperation to move the supplies needed, etc. But small groups... Delightful Targets.
10) Mutants and Monstrousness. Cannibalism is a thing. And there are tribes that have been struck down to small enough numbers they have to Raid to survive. And horrible actions can be just as corrupting as magic without purification. Add to that the curse of the gods and degenerates tend to degenerate into more and more monstrous forms with every evil act. (Note: No racist connotations for corruption, nor ableist ones. Things like cat-eyes and claws, mouths full of too many teeth, horror stuff)