Hyper-linked and cross referenced rendition of the sources, much like doxygen or lxr does it. The desired features were:

cover boot, kernel, networking, library and user level code
cover C, assembler and other relevant languages with syntax highlighting
provide 'what is used where' information, over the whole system
provide hyperlinks for a fast code path analysis, especially across the user,library,kernel interfaces
support to follow the execution logic of a system call starting from user code, through the library layer, through the kernel interface, to the kernel code by just following hyperlinks.