"Linux is not an Operating System?" Come on! That's just pedantic. Honestly, when you hear the word Linux do you automatically think of the kernel by itself or do you think of an entire OS? Personally, I go straight to the latter.
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.