The G5 has some design tweaks for better usability, including a much deeper grip than the G3's.
Don't let Josh's big hands fool you -- the Panasonic Lumix DMC-G5 isn't terribly compact, and in fact is larger than the G3 and is one of the larger full-size ILCs out there. That's partly due to a much deeper grip than its predecessor has.
Here you can see the Panasonic DMC-G5's dSLR-like grip, which I consider an improvement over the earlier model's. It's not a small camera, so trying to shave dimensions off with a shallow grip is kind of silly. You can also see the zoom lever just behind the shutter button, which is designed to work with Panasonic's Power Zoom line of lenses. I think that's another good idea, since I don't like where the zoom lever sits on the lens.
Panasonic rearranged the back controls slightly, adding a programmable AE/AF lock button at the top, which is welcome, and giving the thumb rest a more pronounced feel. However, it's also got the etched labels on the four-way-navigation switch, which I find annoyingly invisible.