I'm just an undergrad, so I'm not really asking this question for myself. This comes more from general curiosity. I know/know of several people who did PhDs from top universities (e.g. Princeton, MIT, Berkeley, etc) but could not go on to be hired by top universities. Many were hired by good and well known universities (e.g. University of Toronto or Haverford College), but not by the absolute cream of the crop.
Based on this, I wonder what it is that qualifies or differentiates someone to research and teach at the very finest universities (I'm thinking Harvard, Princeton, MIT, etc). What's the difference between Princeton PhDs who teach at Princeton/Harvard/etc and Princeton PhDs who teach at Haverford (again, no offence to Haverford - it's an excellent institution - just using it as an example of a school that's good but not quite up there with the New England schools and the likes)? Is it because the former have just engaged in far superior research (and write far better dissertations), or that they have been able to "network" more effectively in the academic community, or something else? What role does luck have to play, if any?