Possibly the best, or most authoritative answer is to read the FreeBSD handbook and see for yourself, so you can discover how FreeBSD differs from other Unix variants like Linux.
http://www.freebsd.org/doc/en_US.ISO...ook/index.html
My impression is that for GUI, there is virtually no difference. In the user land, that is, many Unix commands are identical (at least conceptually) across many Unix environments. But system administration commands and system configurations differ vastly. That is, as you descend towards the system kernel, the more difference you will see.