The new Traitor Heroes will, apparently, be twins of existing heroes, only "evil mirror universe" versions, having at some point rejected the teachings of the Alliance and Horde to join a traitorous faction with the WoW Universe, such as the Scarlet Crusade, the Grimtotems, or the Leper Gnomes, among others. These heroes will have access to a pool of Traitor allies which will include Kael and Vashj, as well as access to various traitor abilities that, like the traitor heroes, will be brutal mirrors of existing abilities (for example, the traitor version of Execute, which destroys a target damaged ally, is called Sudden Death, and will instead destroy a targeted undamaged ally). Finally, each Traitor Hero, in addition to having the traitor allies, will also be able to use ally cards from their betrayed faction. for example, the Grimtotem traitor will be able to coerce the Horde into working for him, and the Leper Gnome traitor will be able to infect Alliance allies to go crazy right along with her. That way, if they can't work new traitor allies into subsequent expansions, traitor heroes will still be able to grab some help.
OK, so next thing you're asking is pretty obvious: Where's the downside? Should I be throwing out all my old cards and working on a traitor deck? There is a catch, yes: Traitor heroes cannot use talents. So you'll have a new pool of abilities and allies to work with, but you'll have to give up Mortal Strike.
I have to say, I'm intrigued. We're talking some awesome lore possibilities for the lore geek and raving Grimtotem fanboy in me, and it should be interesting to see how many players decide that it's worth it to give up talents for the new traitor ability and ally cards. The information's only going to keep coming, as Upper Deck will release 2 new preview articles a day covering various new abilities. It should be a great ride for fans of the TCG.