Quote Originally Posted by Gelatinous Cube
One might say that true Medieval Fuedalism didn't occur until the Knight Social class had already been established.

The thing that makes this heirarchy unique is the religion. There's never been any other example of such an elaborate system of existence, and Christianity is, IMO, the prime ingredient. It was the church that instilled in people the faith to go along with the system, it was the church that proclaimed knights were a valid persuit (despite Thou Shalt Not Kill) if they would go on crusade, it was the church who permeated all social classes and was the glue that held it together.
Still, the hierarchy as you present it was largely an ideal. For example: a middle age village in Holland, Rijswijk, consited in the later middle ages for more then 50% out of people who were legally 'nobles'. The same was true for many other places. However, these people were not all knights, neither did they all own a castle; they led the very same lives as their 'peasant' neighbours, only trying to get tax-exemption from their noble status. At the very same time, more and more 'real nobles' seased to be called knight, simply because they didn't get trough their 'knightly exam' (they didn't fulfill all necessary requirements). You can also see that in this period, still very medieval, more and more nobles go to university (like many burghers) to reach a degree and get accepted into their government, along with these same commoners.
As for the role of feudalism and religion in defining the Middle Ages, I would like to point out that Catholic France before the revolution of 1789 was in many ways still a 'feudal' society.