The Sorting Hat's job is to, well, sort all of the students of Hogwarts into the four Houses. And generally, it does this easily enough by considering the individual students' personalities. But in Book 5, as war with Voldemort hangs on the horizon, the Sorting Hat basically apologizes for separating Hogwarts students in this way. It worries that the house system creates more division between the students than is healthy; it wants the four houses to stand together against Hogwarts' enemies. Oh, the Sorting Hat goes on to sort as usual, since that's what it was created to do, but it expresses its objections in song form before continuing with its task.
And frankly, we agree with the Sorting Hat: separating the kids does create a lot of disunity at Hogwarts. Probably worst of all is the rivalry between Gryffindor and Slytherin. The kids in Gryffindor can't seem to imagine anything good coming out of Slytherin – and indeed, Rowling hasn't shown us any really great examples of decent Slytherins (Professor Snape being a dubious exception). She has commented:
But they're not all – don't think I don't take your point, but – we, the reader, and I as the writer, because I'm leading you all there – you are seeing Slytherin house always from the perspective of Death Eaters' children. They are a small fraction of the total Slytherin population. I'm not saying all the other Slytherins are adorable, but they're certainly not Draco, they're certainly not, you know, Crabbe and Goyle. They're not all like that, that would be too brutal for words, wouldn’t it? (source)
Yet, if it's true that the Slytherins aren't all like that and we're only seeing a small fraction of Slytherin House, where are the other Slytherins? Why don't they join the D.A.? Why do they all seem to flock to the Inquisitorial Squad? Frankly, we do think that sometimes Slytherin House seems "too brutal for words." If Rowling wants us to believe that it is possible to achieve balance between the four houses, we wish that she would show us some model Slytherins as an alternative to the Crabbes and Goyles that seem to populate the Slytherin we actually get to see in the books.