Lots of new features, including multiple framerates, & built in WiFi control.
http://gopro.com/cameras/hd-hero3-black-editionThere's also a firmware update (Protune) for the current Hero 2 that adds higher bitrate, 24 fps (24p coming soon), log-like color space, less sharpening, & NR.
I've just added the WiFi module to the Hero 2, & while it doesn't preview in real time, or at all while recording, it's a great framing aid when rigging the GoPro in odd, or tight locations, & can start/stop recording. It's also much easier to navigate the menu using the phone/tablet app instead of the camera's LCDs.