You're talking about kernel vs userland. That's not base vs non-base. The BSD distributions unintentionally muddy the waters by calling what is an equivalent of a linux distribution a base system and so lose out on the distinction you are drawing.
No, I'm talking about the Operating System vs. Applications.
Drawing the line at the kernel syscall ABI is arbitrary; no other major operating system other than Linux does so.
Consider how difficult it is to ship 3rd-party applications for Linux and all its myriad of distributions; the only thing you can truly rely on is the anemic syscall ABI provided by the kernel (vs. userland).
Hmm, I've never had any problems running something on ubuntu, debian, gentoo, redhat, etc. If it works on one then it works on all the others. If push comes to shove I just install the libraries and compile from source but this is a rare occurence and I've only have had to do that when I've relied on a version of Ruby or Python that has not come pre-packaged with the distribution.
So I don't really know what you're getting at. If you're not talking about kernel vs userland and you're talking instead about ABI compatibility then the ABI is even more stable and requires even less work to port any piece of software from one distribution to another.
