I referred someone to this Stack Exchange. They read the FAQ but still couldn't find what "Interesting tags" and "Ignored tags" were all about. I referred them to http://blog.stackoverflow.com/2008/10/expressing-your-tag-preferences/.
There may be other parts of the site as well that long time Stack Overflow users know about but were never documented outside of blog.stackoverflow.com. They should be added to the FAQ as found (or some reference page that new-comers can find).