VMS FTW!
Fantastically consistent command line interface. Tools that were designed to do the job rather than just thrown together. Sadly the only current hardware OpenVMS runs on is iTanic. So it's days are numbered.
Fine grained multi tasking, and avoiding mutexs. I think BeOS uses message passing to implement inter process communications. In engineering terms, it is the most modern desktop operating system.
It is difficult to get a buffer overflow exploit to run the command line interpreter, when the operating system doesn't have a command line interpreter.
"What man has done, man can aspire to do." -- Jerry Pournelle, about space flight