This year's campaign setting is Dark Sun, and CriticalHits.com has posted up the sample characters (from DnD Experience, I believe). Now I've never really followed Dark Sun, but I've got to admit that after this it's piqued my interest more than the 4e incarnations of Forgotten Realms or Eberron have. It seems that Dark Sun won't offer any new classes, but it does introduce the Sorcer-King Pact Warlock (setting-specific fluff) and the Animist Shaman.
On top of this, players also get to choose a "theme" for their characters, which is basically just a way of further individualizing them. The themes from the previews are Gladiator (the Fighter and Barbarian), Veiled Alliance (Wizard), Templar (Warlock), Wilder (Battlemind), and Elemental Priest (Shaman). As far as I can tell the only benefit of a theme is that it grants an encounter power, which, don't get me wrong, is a great benefit! Gives 1st level characters an additional option. What I like most about these themes is that allows for greater specialization. For example, going with just the PHB you can make a one handed or two handed weapon Fighter. Martial Power introduced Tempest Fighter and Battlerager (which are more distinct fighting styles than simply weapon choice), Martial Power 2 is adding even more craziness (the Brawler Build is what we know of), and now you can add a theme too (well, not exactly "now," as Dark Sun won't be released until August). So a Fighter that started out as just a 2 Handed Weapon Talent Fighter may have been updated as a Battlerager after MP2 because it fit the player's concept better, and now that Battlerager could even pick up a theme (Gladiator, for example). Essentially, I like how it makes the character "imagination" process a bit more immersive.
Dark Sun also introduces 2 new races, the Thri-Kreen (essentially big Psionnic Mantids) and the Mul (Half-Dwarves). The Thri-Kreen is another Dex/Wis race, which apparently even has a speed 7, making it very similar to the Elf. Instead of sleeping, it goes into a Torpor (mechanically, this is like an Eladrin or a Drow's Trance), and once per turn it can draw or sheathe an item/weapon as a free action (multiple arms). The Racial Power is a Melee 1 attack that targets 1, 2, or 3 creatures for damage (the Thri-Kreen essentially mauls the hell out of everyone with its claws). Definitely a solid race (it helps that Dex/Wis are useful stat bumps), but the Elf is still the king of the Dex/Wis races; Wild Step and Elven Accuracy are just too darn impressive to be overshadowed by a racial Quick Draw and a melee attack racial power.
It's not really clear how the Mul works, as it doesn't have a racial encounter power. A Twitter message hints that they have some sort of resistance/save bonus against status effects though. Also, they do get flexible stat bumps: Con and either Str or Wis. They possibly get a bonus healing surge (though these preview characters are notorious for having typos), and they have an ability called Tireless, which basically means that they only have to sleep every 72 hours (they still benefit from extended rests that the party takes, but it basically means that they can stay up all night 2 out of every 3 nights on watch).
Interesting stuff! The only other detail that caught my eye was a new Shaman At-Will, Spirit Infusion. Fluff-wise, your Spirit Companion (SC) possesses an ally and attacks through their body. Mechanically, your SC disappears and the target (ally) with a +2 bonus to the attack roll! Plus with your SC disappearing as part of your standard action, and calling it back is a free action with Sudden Call, re-positioning the SC will be easier than ever! Overall an excellent "grant a friend a free attack" at-will, with by far the best fluff of the bunch!
Other recent news, the 4th PHB3 race has been leaked, and it's called the Shardmind. Pretty much crystalline humanoids. Uhhh, kinda lame but whatever. The 6th class has also been leaked (as well as the mechanics for the Battlemind, which I'm not impressed with): it's a Divine leader called the Runepriest. Seems different enough from the Cleric too! It's a melee leader that can switch between different "Rune States," the two known being "destruction" and "protection." Essentially, depending on which Rune State you're in, your powers will provide either an offensive or a defensive buff. So instead of choosing between offensive and defensive powers, you get both with every power you choose (and it sounds like you can switch Rune States each time you use a power). So a melee leader with a strong focus on buffing, and great flexibility between offensive and defensive buffs. What do you know, it fills it's own niche! Sounds much more fun than a Cleric too, IMO (the flavor just screams Dwarf to me). I'll bet it's not a very good healer, but that's what Clerics are for!