Does Darwin have any features that are specific to it?  Do other Unixe(s) have features that Darwin lacks?
What features does Darwin have that other Unixes don't, or vice versa?
osx;darwin
OS X is the only remaining operating system based on the Mach microkernel which is also still commercially relevant.  There are a few ongoing research projects and obsolescent OSes that no doubt are still being used in production settings on old machines, but nothing you can go out and buy on a new machine today.OS X has the usual assortment of kernel feature incompatibilities that any *ix has.  The biggest one I most recently had to work around is a lack of System V message queues.  (msgget(2) and friends.)  We had to replace our message queue code — which was written for a real System V variant and later ported to Linux — with TCP/IP to get our software to run on OS X.  For our application, the differences between these two IPC methods mattered at the time we made the choice to go with message queues, but due to later architectural changes, it ended up not being a big deal to switch to TCP/IP.