What is the most technically challenging feature of 2009 regulations?

Is it getting the front wing to work well? Or the rear wing? Or the slicks? Or the ban on aero "flicks, flips and flaps"? And dare I mention the "K-word"? I know Ron Dennis and the knowledgeable people will say something along the line of "optimising the overall package", but really, if you are the technical director, what would be the biggest source of headache?