IOKit is actually a considerable chunk of the kernel, particularly if you count the various "families", some of which remain open source. I'd say it's Apple's main contribution to the kernel at the architectural level. It's implemented unlike anything in BSD or Linux, as it uses C++ on the kernel side. (The user space side, IOKitUser, is written in C.) These days even Apple uses it for non-device-related kernel services - you can see what the iokit registry looks like on your system using the "ioreg" command or the IORegistryExplorer or IOJones GUI tools.