The four houses seem to be related to the basic personality types that you find in high school students. Their analogs in a typical American muggle high school (and yes, I’m stereotyping out the wazoo here. This is just generally speaking) would be:
Gryffindors: the jocks who think they can do anything (and tend to get in trouble a lot)
Slytherins: the obnoxious preppies who want to be doctors or lawyers or the president or something like that.
Ravenclaws: the nerds and geeks that nobody pays much attention to (And before anybody gets offended, know that I imagine myself getting sorted into this house
)
Hufflepuffs: everyone else. (And yes, that last sorting song made it clear that Hufflepuff is basically the miscellaneous category.)
BTW, I’ve been reading Mugglenet, and they have some interesting theories. For example, there’s the “Two Book Rule.” Starting in Book 3, the magical skill or item that is crucial to resolving the plot was introduced in a minor way two books before.
Consider:
[ul][li]Prisoner of Azkaban: Animagus ability (introduced in Sorceror’s Stone, through Prof. MacGonagal)[/li][li]Goblet of Fire: Polyjuice potion (introduced in Chamber of Secrets)[/li][li]Order of the Pheonix: Prophesy (introduced in Prisoner of Azkaban)[/ul][/li]
If the pattern holds, then something introduced in GoF will be crucial to the story of Book 6. The cover of the American version suggests that the Penseive (introduced in Book 4) will probably be this item in the next book. As for book 7, it will be something first seen in OotP. Three things immediately suggest themselves: The Veil, the Room of Requirement (referred to in GoF, but not seen until OotP), or the locked room in the Department of Mysteries.
BTW, regarding that Veil, I think that it’s an execution device, a relic from before Britain ended captial punishment. After all, it’s in a room that is identical to the court room where Harry was tried, and both of these rooms are in an old, seldom-used part of the building. And the Veil would seem to be the perfect device for a “humane” execution: it’s quick, it’s not bloody or gruesome, it’s probably painless, and you don’t have to worry about the prisoner failing to die (which might be a significant consideration if there is some sort of Fake Death spell or potion available). You don’t even have to dispose of the body afterwards; you just push the criminal through, and he disappears forever.
Perhaps Harry and Voldemort have their final fight in the Veil Room, and Voldy ends up getting knocked through the veil. Who knows?