Learning to program at the operating system level

I was recently reading some old LWN article[1] about a realtime workshop in which was stated that the academia doesn't prepare CS graduates for working with the development community and more over, they don't know to program at the operating system level. What does programming at the operating system level mean and what does it involve ? How's that different than what's currently happening in software development, what are the constraints and what should an engineer expect when it enters the realm of OSs ? Thanks! [1] http://lwn.net/Articles/355416/