G2 is a little bigger - around four ounces and also bigger in hand. Better AF system, bigger viewfinder. 4 fps max as opposed to 2 fps max for the G1. G2 has a faster top shutter speed - 1/6000th as opposed to 1/2000th. Focusing wheel is located on the front of the G2, but not the G1. G2 also has a "release-priority continuous AF mode"; someone else is probably better explain the differences in actually using the camera.