ArenaNet has released details about Guild Wars 2 Thief on the official site recently after the announcement of the class a few days ago. Overall it appears the profession plays exactly like the original Assassin does with the added benefit of being able to "steal" from enemies to gain environmental weapons depending on the type of enemy they are.
It appears that the Thieves will primarily focus on stealthing in, attacking, and stealthing away quickly. Their resource mechanic seems really interesting, Initiative, which works by giving Thieves 10 points and which regenerate one point per second. They also have Stealth, which functions like your standard Stealth ability, along with the ability to steal which creates an environmental weapon based on the enemy they're fighting.
Some interesting things to pay attention to is that they can dual wield pistols apparently (making them useful at ranged). Also, the level of detail of the sounds in the video is extremely good. So it's really good to take a peek at the videos and see what the Thief is all about!