What are the current open problems in Computer Graphics?
There is a vast amount of open problems in real-time graphics: shadows, aliasing, reflections, global illumination, transparencies (blending order and lighting) etc.

SIGGRAPH annually hosts a course called "Open Problems in Real-Time Rendering", which describes current issues in real-time graphics (mostly game dev industry standpoint). You can find last year's materials here: http://openproblems.realtimerendering.com