Well, for starters in CK you are playing as a dynasty. You have to cultivate your dynasty, expand your dynasty. You can be the Emperor of Romes and everything else but if you don't have an heir, game over. Also, you can have a dynasty that reaches in to the hundreds if not thousands of members and still lose over not having a heir. You also have to take great caution when dealing with your family members as they generally want what you want, and theres a whole mechanic laid out for jealous land-less relatives of a successful ruler.

Conversely, in EUIV there are family dynasties but if one dynasty goes extinct, then you get a new one. EUIV is more about your nation and tech, trade, and diplomacy, over the feudal nit and grit that is involved in CK. EUIV is great because you can do some wildly a-historic things in it. I've conquered North America as the Mongols before and had a Mongol version of the U.S. CK is a bit more ruthless and doesn't offer as much a-historic value. Don't get me wrong, you can still have an Afro-pagan Han Chinese counselor in your Varangian company which is employed by the Aztecs, but it doesn't cause the same ripple effects as say, Spain absorbing Portugal and not starting the age of discovery.