Everything listed in the poll, plus:
*A Strange New World- It's not just Old Time Earth, it's Mad Max ish, full of giant lizards and giant insects.
*Deadly World- Even more so under the 2E rules, Dark Sun is about DEATH. Characters die at random, at the roll of some dice or the choices made: either way they die. This, is of course, lost in most modern games as they have the meta game rule of no character death.
*The Struggle of Life vs Death- Dark Sun is all about the struggle just to stay alive no just vs foes, but also all types of hazards, the environment, and getting food or water. It's very much a survival bookkeeping game. Keeping track of every drop of water and every bit of food, while also watching out of things like heat effects, giant carnivorous plants, and foes. This is all lost on a typical modern game: they would just roll a 10 survival role. And even if a character ran out of water they would just get like "an exhaustion point" with something like "oh no if you get 12 points you will get a negative effect".
*The Savage Wasteland- A few towns, a handful of cities and endless wastelands and wilderness. There are few places to go.
*Oppression- Nearly all of the civilized world is oppressed by massive powerful governments. There is little or no freedom in most of the civilized world.
*Scarcity- Those couple towns and cites are just about the only places on the whole world to get, buy or steal anything. Unless you plan to craft all your own items, you had to stay close to the couple places of civilization. And even so, very little was available.
*Primitive- Nearly no metal or wood, most things are made of stone, obsidian or animal parts like bone. This also means nearly no potion or magic shops, and quite often no real "shops".
*Near Zero Non Heroes- The handful of (player) characters have the huge struggle to just exist and stay alive. There are no real heroes, just characters that live another day. Unlike a lot of other games, the PCs do not easily just become demi gods.
*Dangerous Harmful Magic and Psionics- To the Extreme.