>As an example, on Slackware I can download anything I want and compile and package it. As long as I get all the dependencies there is nothing to stop me.
Well it depends, I'm using Slack too since the beginning. Maybe 90% will compile without any problems, but try the same e.g. with Fedora, Debian etc. - there you will have maybe 70% success or less. With FreeBSD you should find most of the usual software in ports:
http://www.freshports.org/ (at the moment 19515 ports). The rest should be a matter of some tweaking as mentioned above - nothing spectacular but necessary.