2D dock gives you more concentration

As rounded corners point inward and focus your eyes on the content inside an object, any UI objects that are rounded-shape grabs your attention better than those are not.

Rounded corners also point inward and focus your eyes on the content inside an object. A squared edge is the exact opposite – it focuses your eyes on the stuff outside of the object. When you look at the rectangular buttons of iOS 4, you have to work harder to shift your gaze from the outside to the inside of the button.

Now that explains my preference for 2D dock than the 3D dock. As most Mac applications has rounded icons, those apps on 3D dock can often grab your attention while you are doing some task. The 2D dock serves as a boundary between your main task and those rounded-shape app icons.

3D dock has the potential to grab your attention often, as those rounded-shape app icons don't have an object to cover them

2D dock has a boundary, while you are working on some task, it shields your eyes against those rounded-shape app icons.