No multitasking is one that comes to mind. A real time system wants to be small and robust and guarantee a fast response, no stuttering and so on. They're used

No multitasking is one that comes to mind. A real time system wants to be small and robust and guarantee a fast response, no stuttering and so on. They're used in specialized problem domains. But you wouldn't need that on your laptop, you're fine with a sometimes imperceptible delay in exchange for all the goodies in a modern OS. Well, I like my OSes stripped down but no matter how much you reduce they'll never be real time systems.