Crusader Kings is a game of people, not places. It takes a markedly different tack to the usual Paradox approach of putting command of every facet of a mighty empire at your fingertips; instead you have direct control over one man (or woman, but in the agnatic dynasties of medieval Europe it’s overwhelmingly likely to be a man), and while that man may be able to influence a mighty empire through dint of being the person currently wearing the emperor’s purple, the empire itself will be a mishmash of different territories and fiefdoms occupied by feuding nobles who have minds of their own and who constantly have to be kept in line. Personal relationships are brought front and centre while empire management is heavily abstracted; much of the day-to-day running of your kingdom will be in the hands of these autonomous vassals, with your character only having direct control over the small collection of counties which make up his personal demesne. You rely on your vassals for political support, for supplying manpower in the event of a war, and for not plotting to bring your reign to a premature end.
This shift from absolute ruler to manipulative puppetmaster might be what sets Crusader Kings apart from its Paradox siblings, but it’s also something which has provoked some vehement disagreement on whether or not that shift is, overall, a definitive improvement on the Paradox formula.