ABSTRACT: Random browsing of the various essays published on MuggleNet led to a thought about the traits that the four houses are known for. This, in turn, led to comparisons between medieval jobs and their relevance to the four Hogwart's houses.
I was reading through Chantelle's Hogwarts Houses: Analysis of Characteristics" (recommended as a companion piece to this) here on MuggleNet and my thoughts turned to the question 'why would certain houses even have particular traits?'. That question kept bugging me for a while, tugging at the back of my mind (I'm sure that most of you have experienced this at some point in your life). This led to my remembering that the four Founders determined these particular traits, due to their habit of only taking those with desirable traits into their own house.
Now, it is mentioned repeatedly throughout the books that Hogwarts was created by the founders approximately 1000 years ago. This means that the castle was constructed in the late 900's - right in the middle of the Medieval period. Now, castles in that period (being primarily militaristic structures) would have groups of people to oversee all the day to day defenses and keep things operating relatively smoothly. Also, the belief back in those days was that magic was a very real thing, so having a small group of wizards and witches openly using spells was probably encouraged. My thinking is that the school was originally created to train young witches and wizards in both magic and in particular areas of 'castle management', so to speak.
Thus, brave Gryffindors were trained to be knights and men-at-arms, intelligent Ravenclaws became military strategists and leaders, cunning Slytherins were spymasters and assassins and hard-working Hufflepuffs were content to do menial chores that were the foundation of the entire castle (after all, no one ate unless someone cooked their meals for them).
This is also reflected in the artifacts that the various Founders left for future generations to find - Gryffindor's Sword (the classic knight's weapon), Ravenclaw's Diadem (a simplified crown, which is a symbol of leadership), Slytherin's Locket (has a hidden compartment that has been shown to hold a piece of parchment) and Hufflepuff's Cup (which was filled by the servants waiting on people at mealtimes). Perhaps these were the jobs that the Founders had before they created Hogwarts (which would go some way towards explaining why these particular items became known as the Founder's Artifacts - sentimental attachments on the part of the founders).
As the title says, this is just a theory I came up with after reading Chantelle's essay, so you can take it or leave it as you like. If you come up with your own theories on this subject, feel free to publish it here, as I would enjoy reading them.