Somehow I think the game is a little bit larger than Apple finally getting a server OS right.
Nope, that's about it. I was always a strong critic of the architecture of OS 9 and earlier. It had no preemptive multitasking, no protected memory, no SMP. Why should a developer at the turn of the millennium have to use special tools to make sure he doesn't overwrite another process' memory or get too greedy with the processor time (certain real-time applications excepted)? Why should he have to manually apportion processor time in a master/slave configuration? Windows NT at least had that figured out years earlier, and System/360 had protected memory in the 60s.
OS X is UNIX, and thus gets the power and flexibility. Apple improved many of the UNIX tools, added a few of its own, and made the whole thing easy to use. Also important is that Apple decided not to use CALs.