TW has updated their wiki page of factions. Unfortunately, at the moment it is broken (announcement from Sega is here). Posting link for future reference, when they get their server situation sorted.
I was able to pull some text, however, despite the page's current crippled condition. To wit:
Ranging from the Spanish coast to the far-flung, exotic kingdoms of the east, the campaign map for Total War: ROME II™ is breath-taking in scope, and a study in detail and variety. This page is your source for background information about each of the playable factions: their starting position on the campaign map, their civic and military focus, and some of their key battlefield units.
The playable factions represent key powers within the Greco-Roman, Barbarian, and Eastern cultures, and each offers a notably different and deeper form of gameplay experience from those in previous Total War games. Each brings unique commercial, military and political strengths, its own agents and political system, and three tech trees representing civic, military and engineering disciplines. Each will have its own dilemmas to face and different styles of army to manage. Some factions are split into a number of playable families, which bring further unique benefits to their base faction traits.
Players will need to work with – and in certain circumstances against – their faction’s internal political system. You’ll direct the actions of famous characters from history, and if they’re not out in the field commanding your armies, they’ll be politicking in the senate house, or its cultural equivalent. Players would be wise to keep an eye on such individuals…
Some factions rely more on mercenaries for their military might; some prefer to train their own. Some rely on client-nations or vassals to boost their economic growth, while others are more trade-focussed. Whichever you choose, each faction brings a completely different gameplay experience to Total War: ROME II™.
Page seems to be up now, looks like the only faction revealed is ... Rome. Sigh. Betcha didn't see that coming, what with the game called "Rome II."