"GNU will be able to run Unix programs, but will not be identical to Unix. We will make all improvements that are convenient, based on our experience with other operating systems. In particular, we plan to have longer filenames, file version numbers, a crashproof file system, filename completion perhaps, terminal-independent display support, [...]

Linux has, eventually, started to fulfil the promise: technologies like cgroups, dm, uevents, kdbus, alsa..., and the respective userspace: systemd, dmraid, lvm, udev, pulseaudio, show that GNU/Linux is not UNIX but better in some respects.