"Slingshot" is a reference to the biblical tale of David and Goliath, in which [Spoiler Alert] underdog David takes down the giant Goliath by throwing a rock at his head and then decapitating him while he's down. In the first chunk of XCOM DLC, we'll
be given the opportunity to go Old Testament on a massive alien ship hovering over China in a three-mission story arc with a potentially rich reward at the end.
The first two of the three Slingshot missions will take place on the ground in China, on identifiably Chinese maps. These will be the first XCOM maps that aren't geographically ambiguous, which is a great step forward in making it feel like we're actually responding to a crisis in China rather than Montana. Sadly, Firaxis tells me these maps won't pop up elsewhere in random encounters due to a commitment to not repeating maps in a single game.
Dissent in the Ranks
Also, for the first time in XCOM there'll be a named character on the tactical field (other than VIPs in escort missions) who you won't be able to rename or customize his look due to his unique face and voice actor. All we know about him at this stage is that he'll be a Chinese Triad gang member who'll be a VIP in the first stage of Slingshot, but will then join XCOM as a mortal, disposable Heavy with greater-than-normal stats (but no unique abilities).
When Slingshot's missions are all over, you'll earn a shortcut through the tech tree that'll allow you to research the powerful Blaster Launcher and Fusion Lance technologies -- they'll still be very expensive to research and build, but you won't have to shoot down an alien battleship to get them. I think that's a good thing, as in the normal campaign the Blaster Launcher only becomes available in the leadup to the final mission, which only gives us a few missions to mess around with it before the end. Firaxis says it's not expected to unbalance players' progress, since going in early will be risky move with a big payoff, and going in late will be a cakewalk for a reward you'd have gotten soon anyway.
While there won't be any new equipment, abilities, or aliens added, there will be alternate looks for Titan, Ghost, Archangel, and Psi armors, plus additional hairstyles and helmets to give your squads a little more visual variety. Unfortunately you won't be able to take that Chinese voice and apply it to other Chinese soldiers, as it's too specific to his character. Firaxis says it's aware of gamers' desire for accented voices for each of the countries involved, but because of the logistics involved it's not something it's planning on right now.
Source of the info--> http://uk.pc.gamespy.com/pc/xcom-enemy-unknown/1226472p1.html
There still isn't a release date or price for Slingshot, nor any information regarding the second DLC pack. However 2K have announced that there will be a third DLC pack released next year.