Rendering real-time graphics comes with more challenges than just slapping a 3D object onto an AR marker. Real life comes with some complicated visual effects like mirrors, shadows and objects overlapping each other. For AR to be realistic, these challenges need to be overcome.
David Cochard shows us a couple of videos demonstrating his solutions to occlusions, shadows and mirrors. More details on how he accomplishes this task, and other videos, can be found on his site.