I can't believe a HP thread on Babycenter got to the fifth page before I saw it...
Someone asked why Hogwarts kept the Slytherin house around, and though a lot of what I would have said had already been said, (aside from a joke that goes something like "Where would our democracy be with out all the politicians?" ) I still managed to babble on for 700 words or so, and I might as well save the thoughts here, as they get spiritual at the end.
When one looks at the basic drives behind each of the houses, one sees that while some may think the other unfavorable, they are all worthy endeavors.
One must also consider that each person is in a house either through choice or because they mostly identify with one ideal or another. No one is all Gryffindor. Nevil Longbottom would have been an excellent Hufflepuff. Hermione obviously would have done well in Ravenclaw. Harry's drive to succeed would have lead him far in Slytherin. Ron is probably the only one I think is roundly Gryffindor, and look how many character flaws that beloved Ginger has.
For the text of the sorting hat songs, see here.
You also have to consider that the sorting hat is biased. Not only in the fact that was plucked from Gryffindor's head and had loyalty, in the fact that JKR herself identifies with Gryffindor.
Gryffindors are described using: brave, daring, nerve, chivalry. They also tend to rush in with out thinking, which makes the rest of us wonder about their sense, as much as we love them.
Ravenclaws are described using: sharp mind, of wit and learning, ready mind. It is demonstrated again and again that Ravenclaws are more interested in ideas and learning than much else. They are also rumored to all be beautiful... However, when one has one's head in the abstract and ethereal, well let's just say it's easy to lose touch with the things that matter. Because of my thirst for knowledge I would fit well in this house, but as it isn't my first priority, I'm sorted in to Hufflepuff.
Hufflepuffs are described using: just, loyal, patient, hard working, and equal opportunity educators. I can't say enough for my house, I think we are the best kind of people, but I also, as a Hufflepuff, see huge amounts of value in the traits of the other houses. The drawback to being a (most excellent) Hufflepuff? We worry about others so much and try so hard to be fair that we seem wishy-washy and hardly ever strike when the iron is hot... or at all. We make the world go round, but you won't often find us out saving the world, unless you count service projects, then we are sooooo there.
Slytherins are described as: cunning and ambitious. Slytherins network like none other. They see ways of getting things done that would simply never occur to anyone else. They see the layers of relationships. They spend a lot of time thinking like a chess champion, how one move affects all else. Now, this often makes us think of secret combinations, but just because something isn't out in the open, doesn't mean it is wrong. Just because someone wants to make a difference, doesn't mean they are evil. In fact, I really think that Slytherins should be in charge of international relations, because they can think like that and it just boggles my Hufflepuff with Ravenclaw leanings mind.
Now one other thing you should note is that Gryffindor and Slytherin started out as friends, and that it was the years and years of competition between the houses that caused the rift.
Which just goes to show you once again that peacemaking cough cough Hufflepuff cough cough ways are best. If division had not been allowed to creep in, no one would ever have put a Basilisk in the basement.
On a completely different note, and I hope this doesn't sound sacrilegious, because I don't mean it that way, I see in Jesus Christ the embodiment of all of these virtues and none of the side effects. He was brave without blundering, intelligent without being disconnected, fair without being ineffective, and perceptive and able to see the layers in a situation without using it for his own glory.
So if you don't like Slytherins, it's just because you don't understand them, and may need to be a bit more like them.