And FreeBSD has all the reason and effort to sell the linuxes?

The main reason that the Linuxes are incompatible would be that they use different styles. Different styles that are not decently analigous to this thread.

The one that screams to me the most is the Package formats. You have RPMs, .deb, source .tgz, and the ports system. We are not here to argue which is the best, but generally, most will agree that they prefer one above the others (well, of the linux users anyways). This is something that splits linux but cannot really be applied to CPAN.

The next one is how they handle initd and runlevels. Some do it BSD style. Others do it System V style. Something that would not bother CPAN.

As for documentation... that is the maintainer of the project's job, not the people distributing the project. So unless the people making the distro are making custom software to go with that distro, there really is no reason for them to document it.

However, there are other differences between Linux distros, that show this point. Keep in mind, that I will generally use any linux that does not make use of RPMs, so I tend to be more blind to differences. How do I know I am blind? I let a 'windows expert' (yes, we all know what that means) use my system, and he did not think KDE resembled anything he touched before.

Differences are there that we don't notice, and while certain differences may cause problems in one area, they will be benign in others.