When we talk about free software, we usually refer to the free software licenses. We also need relief from software patents, so our freedom is not restricted by them. But there is a third type of freedom we need, and that's user freedom.
Expert users don't take a system as it is. They like to change the configuration, and they want to run the software that works best for them. That includes window managers as well as your favourite text editor. But even on a GNU/Linux system consisting only of free software, you can not easily use the filesystem format, network protocol or binary format you want without special privileges. In traditional unix systems, user freedom is severly restricted by the system administrator.
The Hurd removes these restrictions from the user. It provides an user extensible system framework without giving up POSIX compatibility and the UNIX security model.
However the OSHS goes even further. It avoids the need for the POSIX compatibility completely. You, the user, can choose whether your OSHS installation will be POSIX compatible or not. You can decide, which standard will be supported by your operating system and therefore you can decide that your particular operating system installation will not support standards that are not needed for that operating system installation.
Most users will choose the POSIX compatibility. But under some circumstances the POSIX compatibility is not desired. An example is a controlling computer of a HI-FI system. A typical HI-FI system consists of a FM tuner, a CD player, a cassette recorder (and some other audio generator devices), the speakers and the computer that controlls all this stuff and provides a nice user interface so the HI-FI system can be used even by computer antitalents. In such a system, there is no need to have profiling timers, POSIX compatible shell, tty support or POSIX sockets available. These features only consume memory space and makes the system more expensive.
Another examples of such devices are a video cassette recorder, a DVD player or an air pollution measuring station. All these devices need only a very limited set of operating system capabilities to support their functions. Using a classical POSIX compatible operating system for the control computers of these devices is just like killing sparrows by thermonuclear bombs. With OSHS you can strip the operating system down to contain only the necessary functionality.