For those systemd users, it seems not as a big of a deal as "the register" poster might imply. In the end it'll depend if the new soft reboot is called or not (it's doubtful distros will change default behavior either).
Systemd's "soft reboot" has nothing to do with Windows' "fast startup". Those are two completely different concepts for two completely different use cases.
... I feel like loading the boot sector of a disk and jumping into the boot loader would be a more useful feature than whatever this is. I usually reboot to switch os or upgrade the kernel. Userspace cleanup is already pretty easy. How do zombie processes or open files fare with this scheme? Crashed drivers? Why replace the root fs?