Gliese 581g is huge. Its mass is 3-4 times that of Earth. The newly discovered planet is much closer to the size of Earth, which is what the headline is all about. (Of course, mass and radius variations in this range have little to do with habitability, since both planets probably have enough gravity to hold onto an atmosphere.)
Gliese 581g has a surface gravity estimated to be between 1.1 and 1.7 times Earth's gravity.
At 1.1x, it's possibly livable for humans, making it more interesting; 1.7x is probably too rough without some new technology or adaptations.
Also, Gliese 581g is unconfirmed, and there is a lot of doubt about whether it exists.