I went through my school's version of this class on xv6, and wow, what a trip. After thinking hard and working hard on things I never had a solid grasp on - filesystems, virtual memory, interrupts, multitasking - I now appreciate and understand things going on after a call to fork() or exec().

For my final project I implemented a simple threading library based on the interface that pthread() uses. It's amazing how beautifully simple kernels can be.