Userspace typically makes people assume a security model, Ring 0/3, etc. I think of an Operating System as something that provides a Hardware Abstraction Layer

Userspace typically makes people assume a security model, Ring 0/3, etc. I think of an Operating System as something that provides a Hardware Abstraction Layer (HAL) and system calls to take advantage of that hardware. Drawing on the screen without knowing the details of the video card, file io without caring about IDE/SCSI, etc. If it runs other programs but those programs still need to take direct control of the hardware then the OS is just a glorified boot loader.