One type of clue that's fallen out of favour, according to a book I read about it a while back, are those which require knowledge of, for example, the classics. It used to be that you needed a pretty good private education, but as English speaking has become more international, and as fewer and fewer English speakers (as a percentage) have that sort of education, those clues are less likely to be useful.
Different setters take different approaches; it's why the papers almost almost name the setters, and why it's a big deal when one retires as they're sort of unique.
