When I was in High School, the original Dark Sun 2e box came out after much (for the time) fanfare. I couldn't wait to get my grubby little paws on it, and once I did, it started a love that's lasted for almost 20 years.
Sure, at the time, I was largely interested in the powergamey stuff - the wild talents, the four-attack-per-round thri-kreen, the Strength 24 half-giants, and so on. But over time, as more outstanding supplements came out, I was enthralled by the setting itself. It's bizarre and unique and nothing quite like it has ever been released before or since. I mean, a desert world with unique and deadly flora and fauna, where psionics are commonplace, and powerful wizard-psions rule city-states while turning into dragons? It was insane and utterly captivating. And then TSR basically blew up the setting by throwing a common-for-the-90's metaplot at it. (A metaplot, fwiw, which my players and I completely ignored.)
Anyway, I've been anticipating Dark Sun 4e since it was announced a year ago. The book has been well worth the wait, and sets a new high water mark both for 4e settings (admittedly not a high bar at the moment) and 4e books in general.
Okay, so what can a fan of the 2e setting expect? Well, the most important thing to remember is that, just like Dark Sun 2e took the iconic parts of the 2e setting, Mad-Maxed them up, and added spiky bits, Dark Sun 4e does the same to the iconic parts of the 4e setting. So there's some new stuff - Tieflings, Eladrin, and Dragonborn for example. Much like the other races, these are thrown through the Athasian blender. Tieflings are "desert devils" - cruel, bloodthirsty raiders in service to demons. Eladrin are xenophobic, mage-hating psions clinging to the last vestiges of the rapidly-dying Land Beyond the Wind. Dragonborn are the Dray - which were around in 2e, reskinned. Half-Giants simply use the Goliath mechanics, which is fitting. Setting favorites Muls and Thri-Kreen make their 4e debut. There are very few other races still around, unless you wheedle your way into them with your DM.
By default, the gods are dead and gone. That means no divine classes, either. This was a jump a lot of people didn't expect them to make, but I'm glad they did. Taking the place of 2e's Elemental Priests are new options both for Shamans and for all other classes as well.
Mechanically, the biggest innovation for Dark Sun are Character Themes. More or less, these are paragon paths you take at 1st level; they sit on top of your class, and tie your character further into the setting. Among these options are Gladiator, Elemental Priest, Templar, Wilder, Noble Adept, Dune Trader, and Athasian Minstrel. They give you a handy Encounter power, and down the road, you can pick a series of Powers from your Theme instead of your class.
It also takes a major step back from magic items. An inherent bonus system, similar to the one from DMG2, is presented as the default, thank goodness. This makes the characters "work" mechanically, even without magical gear at all.
OK, enough about mechanics. The setting itself? Gorgeous. WotC listened to fans, and reset the timeline to just after the death of Kalak. This means no Cerulean Storm, no dead Dragon, etc. It's awesome. Each City-State gets a few pages all to itself, and every one gets a nice map. There's a lengthy section on adventuring in Athas, with (harsh and deadly) rules for travel and desert survival.
All in all, it takes all the stuff I love about Dark Sun and all the stuff I love about 4e, and puts them in a neat little package which my players are already clamoring for me to run.
I recommend this book to anyone who's a fan of Dark Sun of any edition, any DM looking for something other than typical fantasy, and anyone who was on the fence about 4e and needed something awesome to convince them to take the jump.
One note - if you are new to 4e, and Dark Sun has convinced you to take a swing at it, I'd strongly suggest a DDI subscription. It will let you get all the character options included in Dark Sun for an intensely low price.