Looks great at 120Hz but I notice a bit of motion blur at 60Hz... I notice that the blur actually is just a trick of the eye though when moving fast, because if I change focus and carefully look at anything "blurred" it's immediately no longer blurry. When I play at 120Hz though it looks totally fine.
I'd say, like the others have suggested, it's a bit of an optical illusion because things are moving rather quickly.
Neat game btw, and cute graphics.