The new content is called "Bring Down the Sky." It is a new explorable world which features an extremist group of Batarians. Apparently, these Batarians have highjacked an asteroid and are trying to hit a colonized planet with it. (Don't tell Fox News about the new race. They'll just think it's someone else to have sex with.)
The new content adds about 90 minutes of gameplay and one Gamerscore achievement worth 50 points. I'm looking forward to it!