> What are the big architectural differences or cool features not in Linux?
Darwin is based on XNU from NeXT, which itself is comprised of OSFMK Mach microkernel but with many of the things pulled into the kernel itself for performance. A lot of the stuff with Mach tasks/ports/messages is still there.
> Also, how hard is it to upstream changes/get drivers in mainline?
You can write your own using IOKit but they're not going to be in the mainline tree, because Apple.
> Darwin is based on XNU from NeXT, which itself is comprised of OSFMK Mach microkernel but with many of the things pulled into the kernel itself for performance.
And amusingly, the syscall interface from FreeBSD 4 or so.
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.
> What are the big architectural differences or cool features not in Linux?
Darwin is based on XNU from NeXT, which itself is comprised of OSFMK Mach microkernel but with many of the things pulled into the kernel itself for performance. A lot of the stuff with Mach tasks/ports/messages is still there.
> Also, how hard is it to upstream changes/get drivers in mainline?
You can write your own using IOKit but they're not going to be in the mainline tree, because Apple.