Shape and the lack of a scroll wheel:
http://www.quakelive.com/forum/showt...th-a-ballmouse
my accuracy's gone even higher since I made that demo, partially due to me learning how to really refurbish my wingmen. I'm sure that if I had an optical or nice laser sensor (which I plan to eventually implant into my wingman shell), it'd be a bit better, but in my experience, the important factors are mouse shape, smoothness of mousepad/mousefeet interaction, increasing USB polling rate to 500hz, and having relatively well maintained rollers. Perhaps my mind will be blown once I actually make the transition - The only two non-ballmice i've quaked with are the logitech g9x and g600, both of which I used for about 20 min each. The tracking is definitely impressive.