It typically has less to do with performance or stability than with ease of use. While Linux is piling on features, BSD focuses on clean design and quality of code and documentation. Linux is "do it now," BSD is "do it right."
I hate to press you on this point -- the wreckage of many a
flame war has barely had time to grow moss -- but can you point
to a specific feature? I assume you mean OS level features --
system calls and such.
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.