-Interleave the execution of the number of processes to maximize processor utilization while providing reasonable response time

To be able to load applications, provide for multitasking either through timeslice or priority/interrupt mechanisms, you want to be able to detect a deadlock/race condition and take an appropriate action to resolve the issue, you want to have interprocess communication of some kind, you want to provide communication services for different types of hardware and logical communication consructs, you want to provide "virtual memory" for applications that are larger than physical memory, you want to be able to send a "signal" to process that are waiting for events such as serial I/O and you want to provide user security so that users can have a workspace that cannot be "hacked". Above all, you need to provide access limitations so that viruses cannot gain control. Read about UNIX/Linux. In my opinion it's about as close to a perfect OS as can be found.