Hmm... well, BSD init is easier to use than SysV init. The ports system is also known for its simplicity. The source code is contained in a single CVS tree and the entire OS can be built with a few commands. The man pages and handbook are consistent and well written.

Much of the difference is subjective. FreeBSD feels coherently designed, whereas Linux is a patchwork of programs written by different people with different styles and philosophies.

I don't know a lot about Gentoo (most of my experience is with Debian and Slackware) but from what I understand, its purpose is to be as flexible as possible. That's a legitimate goal, but it seems to conflict with ease of use.