For starters, January will bring a new flashpoint to the game as well as an expanded operation instance. The former "takes place on a brand new world and involves some pretty epic battles." The latter "takes the existing Karagga's palace scenario and more than triples it in size."
Ohlen says that's just the beginning, too, and BioWare plans on releasing similar new content with regularity. Finally, players concerned over the company's ability to add new story (and voiceover content) will want to check out writer Alexander Freed's post on the official forums. In a nutshell, he says not to worry, as BioWare does have voice actors under contract and can proceed with development without adding much time to the content creation process.
Finally, our sister site reports that TOR has topped one million registered players and over 28 million hours played!
[Thanks to Scratches for the tip!]