A general purpose machine is all about maximising thruput, a RTOS is all about meeting deadlines, this impacts everything from scheduling (and its interaction with IO and VM subsytems to process migration between cores to locking strategies.
For example most RTOS do priority inheretence to ensure that a high priority task cannot be blocked for arbitary time by conteding on a lock held by a low priority process while a medium priority process hogs the CPU, this sort of thing has far reaching implications and is usually not a feature of general purpose operating systems.
Regards, Dan.