I love my Mac, but FreeBSD/Linux is the better server.
I like having control over when I update packages, I don't want to have to rely on Apple to push an update and "certify" a package (and then be forced to restart the server after this patch/update has been applied).
I want to be able to work independently of the GUI (remotely). While you can do this in OS X Server, sometimes the GUI gets in the way and overrides settings you have entered in the command line (the dorky MySQL Admin thing in OS X 10.3 Server is a good example of this).
Finally, I want real package management