Some thoughts:
- It seems like the lighting doesn't fit the direction of the shadows
- The cubes move very slowly in comparism to the rotation-speed
- Don't let the cubes intersect each other
- The checker appears very flat (doesn't fit the depth of the scene)
- What about a bit of transparency of the cubes?
- The cubes appear to have no weight (you could use a damping-factor on rotation & bouncing)
- Select a position for the light-source that makes the shadow look more interesting (don't project straight down)
- How about making the cubes shadow each other?
- Let an artist pick the colours
