"All those portability claims and that there somehow good: you're just doing things for lowest common denominator. Requiring a huge amount of effort to change or develop low level bits across BSD and Linux."
I don't even know what you're trying to say here.
Regarding your question: GNOME does a lot of infrastructural work (freedesktop.org, and other things). A lot of the times things are just not available. Thing e.g. of colord (colour management), PackageKit, hal (dead :-P), ConsoleKit (dead :-P), etc. Making that work on Linux is one thing (already loads of differences). Making that work for others as well, while sometimes there is not much available only increases the time you have to spend on things.