Vast portions of code that makes it usable are missing; as someone above stated, Aqua is missing as well as core items like disk image mounting etc.

I tried many years ago to use Darwin in the "haha I can run Mac OSX but not actually buy a Mac" sort of idea - it was an exercise in hard work (as I hadn't a clue what I was doing) and what I did managed to get to boot wasn't massively useful for "real" work (a CLI with no windowing layer) like development etc. or anything you do day to day.

Quite interesting nonetheless, but not massively useful.