I find annoying when a project identifies itself as an "Operating System" that I didn't know about... until now, because I suspect that it may be a Linux distribution and not a new OS, but I can't tell from the main page.
I understand why (tying to avoid the "yet another Linux distro" stigma), but when you go to "What is Qubes OS?" in https://www.qubes-os.org/intro/ ; they fail to mention that is Linux based.
What Qubes is is a Xen hypervisor (as I understand it, hypervisor independence is a work in progress) with a particular configuration and set of tools to serve as plumbing between various virtual machines and the dom0 host.
They distribute Fedora, Debian and Whonix domU guest images (and the dom0 host that is installed is Fedora), but you're free to install BSD, Windows, or whatever else you can get to run as a VM guest.
It's a bit weird that they don't mention Linux on that page at all, but I agree with them that labeling it as a "Linux distribution" isn't very useful. From a user perspective they could swap the Dom0 operating system, where most of the important bits happen, out with something else and ideally that wouldn't be an important change.
For the guests that actually run the software you use directly, they provide templates for multiple Linux distros (and don't have their own brand) and support Windows as well, so I don't think the application-level is seen as part of it. Whereas I'd see "provides a more or less curated selection of applications + configuration" as an important part of a Linux distro.
While I entirely agree with you wrt the kernel, the separation and isolation bits they do is new and novel from a general purpose operating system's standpoint. Also, Xen really is a small microkernel that runs ontop of Linux, so their comments on the separation being more secure than if they used KVM actually do have merit. If you want something similar that actually is a different operating system, checkout the Muen Separation Kernel (which is pretty neat tech).
I understand why (tying to avoid the "yet another Linux distro" stigma), but when you go to "What is Qubes OS?" in https://www.qubes-os.org/intro/ ; they fail to mention that is Linux based.
Well, to be fair this one has a FAQ entry: https://www.qubes-os.org/doc/user-faq/#is-qubes-just-another... ; although they try to understate that they use Linux.
If they want me to trust them, they're doing it wrong.