Linux is not an Operating System
tech-no-media.com
tech-no-media.com
The kernel, of course. GNU/Linux is the most popular OS based on Linux, but there are other OSes based on it (usually for embedded systems, where the GNU userspace is too large).
Silly of me to have asked the question in the first place then? I'm aware of many differences between distributions. Still, I wouldn't hasten to claim that the kernel is the only similarity between many or even most distros.
Personally I disagree: I think 'Linux' is actually multiple, fragmented OSs, in the same way that 'BSD' is.