I'd really like to see one of these get written one day from a couple of deeper perspectives... The difference between Linux and FreeBSD is bigger than just "they are variants on a theme", even if this is true at the sysadmin level, and even from the application programming point of view.

At a deeper level - things like "non-posix" syscalls vary a lot, and the subsystems they interact with are wildly different. Some of it is again variations on a theme - a special syscall vs an ioctl, or what not, but the way those subsystems interact starts becoming very interesting at a systems architecture level. For instance, I might want to write a specialized networking tool mostly in userland in linux, via nf_queue, whereas in FreeBSD I might want to split the operations between the kernel (via netgraph) and userland (via the pf equivalent to nf_queue).

It would be nice to see a nice write-up, or survey of writeups of the differences at that level.