The World of Warcraft: Cataclysm expansion will bring two new playable races to the game, the Goblins and the Worgen. A panel discussion at BlizzCon 2009 gave specifics on each race's unique abilities.
The goblin race, who will join the Horde, can be any class except Paladins or Druids. All goblins will wear a rocket belt that allows them to use one of two special abilities: a 20 yard jump, or a rocket volley PvP attack with a 30 yard range. Every half hour, they'll also be able to summon a hobgoblin servant that gives them remote access to their bank account.
Goblins will also have a number of passive abilities. They get a bonus to their alchemy skill and can gain more health/mana from potions than normal if they choose alchemy as one of their professions. They'll also receive Exalted level discounts from merchants no matter what their reputation with a specific faction is. The last goblin bonus should make you power gamers stroke your chins: a permanent 1% speed boost when attacking or casting spells.
Worgen, the new Alliance race, can be any class except for Paladin or Shaman. They are essentially werewolves and will be able to switch between their true form (i.e. furry beast of doom) and human form at will. There's no difference in stats between the two, but Worgen players will switch to their true form automatically when in combat. Every three minutes, they can activate an ability called Darkflight that boosts their movement speed by 70%.
In terms of passive abilities, Worgen will have a 1% damage increase, reduced curse/disease duration. Their favored profession is Skinning and, in addition to receiving a straight-up bonus to the skill, Worgen will be able to skin corpses quicker.
It's worth pointing out that all of these abilities are subject to change. Once they launch the Cataclysm beta, they'll find out whether these abilities are in need of "nerfing" (weakening) or "buffing" (enhancing) to keep the game balanced.
Update: Here are the walkthrough videos for Lost Isles and Gilneas, the starting areas for the Goblins and Worgen respectively.