Did you only read a few words of my post? How about binary (or even source code) compatibility with everything that is not POSIX (which is most of what a modern application needs), desktop environments, the very ways that applications interact with the user? (where files and data are stored etc.) What else is there? Sure, man 3 printf may have had the same interface for 30 years, but it's disingenuous to use that to claim that it's "the most backwards compatible platforms to build anything on".

What's even more funny is that you use "Unix" instead of "Linux", implying that software written for one Unix would magically work with others (I mean, that would be case if all was backwards compatible, right?). Have you ever written software for more than one Unix, even disregarding historical versions? The most painful concoctions needed to develop for several Unixy OS's (like, say, autotools) exist for the exact reason that they're not compatible at all. While at the same time, I've written code for Win98 almost 15 years ago whose binaries still run, without as much as changing a software setting, on Windows 7 (and probably 8 although I haven't tried) today. (again, there are plenty of things to critisize Windows for, but backward compatibility is not one of them).

"Of course things like config files change"

Yeah, that's like saying "of course a 2012 BMW is different in a few minor ways from a Model T, but essentially they're just machines with engines and wheels." Eh, yeah, sure.