I disagree, the problem with 3D is that you are not allowed to focus your eyes anywhere you want in the 3D landscape presented to you. If the depth of field in a scene is substantial, the director will have decided for you what is in focus and what is out of focus. This is not how our eyes are used to work; they appreciate their autonomy in focusing. But I assume that technology-wise, it would be much more difficult to implement such a thing. You would need Lytro-type cameras, sensors on screen to track your retina to understand where you are focusing your attention, and... good luck adjusting the focus individually for each viewer, on the same screen.