Western Medieval Catholic kings and nobility in Britain, France, Spain and Portugal, Italy, Germany and Austria share certain distinctive common features that can be summed under the term of vassalage that are less present in Hungary and Poland, possibly also in Scandinavia. I am referring to the tradition of local autonomy of noble fiefs, integrated withing a pyramid of vassalage but very stable and distinct for hundreds of years. Many such fiefs operated as separate states or as states within states - not far from a federative structure, within which the power of kings was tempered by that of his nobles, and that of the great lords by the power of their vassals.
In Poland and Hungary kingship was not hereditary, but elective. Why this difference? Is a Germanic versus a non-Germanic order at play here? What was the situation in Scandinavia?
"Modern states" means in the Western part of Europe the dissolution of this system and the centralization of kingdoms ("absolutism"). But a different kind of absolutism was typical in Eastern Europe namely in the Orthodox Christian realm (Russia, Bulgaria, Serbia, Wallachia, Moldavia) were a system of Byzantine origin seems to have prevailed. In some cases nobles had no stable fiefs (wasn't this the case in Poland too?) and the King/Prince had absolute power on these (as well as on the nobility status). In some cases the leadership was not hereditary but the power was absolutist. Was this really a Byzantine (or even Roman) feature or is there a different origin at play?
Other interesting aspects here may be the different roles played by religion - the presence and absence of Catholicism (in the differences between Catholic and Orthodox rulers and nobility) and the initial rivalry between the Pope and the (Western) Emperor (can that count in explaining the difference between Germany and the more centralized kingdoms?).