The GH2 was an expensive consumer camera with some really nice video features; the GH3 is an updated version designed to meet the needs of the indie and pro videographers who somewhat unexpectedly gravitated to the GH2.
It's practically an entirely new camera: new body, new sensor, new autofocus system, new OLED LCD and EVF, new video codec, useful new still and video features, and Wi-Fi connectivity. The body's now dust-and-splash sealed over a magnesium alloy chassis, with a more streamlined control layout. According to Panasonic, the sensor plus the new version of the image-processing engine have an optimized low-pass filter for reduced moiré, an expanded dynamic range with the company's most recent noise-reduction algorithms, and improvements in white balance for more accurate color.
Source
