**This article includes minor SPOILERS and quatations from the books, so in case you have been living in a hole and don't know the ending yet, you might reconsider reading this.
Notice it is also purely opinional and any nerd-rant-ish style is not meant to offened.**

Since sorting has been introduced very early in the book, Harry learns about it from Draco before even going to Hogwarts, it seems like inseperatable part of Harry Potter Universe, a static thing that took place since the founding of our favourite Magical School (it is even in the Hogwarts' crest).
It was the base of some subplots, the most notably the Quiddich teams and the general rivaly between houses.
A lot of readers guessed/took quizes to determine to which house they belong.

You might belong in Griffindor,
Where dwell the brave at heart,
Their daring, nerve and chivarly
Set Griffindors apart;
You might belong in Hufflepuff,
Where they are just and loyal,
Those patient Hufflepuffs are true
And unafraid of toil;
Or yet in wise old Rawenclaw,
If you've a ready mind,
Where those of wit and learning,
Will always find their kind:
Or perhaps in Slytherin
You'll make your real friends,
Those cunning folk use any means
To achieve their ends.
- Sorting Hat in Philosopher's Stone

By the end of the first book we pretty much got the idea of an average Griffindor (intelligent, brave and world-saving, with great sense of humour and born leader), Slytherin (arrogant, Muggle-hating, world-destroying, snoby and pureblooded), Ravenclaw (smart, but without world-saving complex) and Hufflepuff (everyone else/ WTF is Hufflepuff?).
This image is based on the presentation (or the lack of it) - until the fifth book most of main protagonists were from Griffindor (Harry, Hermione, all Weaselys, Neville,...) and even in the last three books, they stil prevailed, which is probably the main reason that most readers wishes to be in Griffindor; Slytherin having (almost) all the bad guys in the series gave it a smaller, but stil noticable fandom; Ravenclaw (with the introduction of Luna) managed to get a reputation of a cool (not too nerdy) house and Hufflepuff is well, Hufflepuff (the characters from this house are underrepresented and mostly minor and because Griffindors were also loyal and just, which were supposed to be Hufflepuff's qualities, this house scarcly has any fans).


The idea of Hogwarts without the houses was present in the last book twice, by two completely different characters - Dumbledore and Voldemort.

You are a braver man by far than Igor Karkaroff. You know, I sometimes think we sort too soon ...
- Dumbledore to Snape in The Prince's Tale, Deathly Hallows

There will be no more Sorting at Hogwarts School. There will be no more houses. The emblem, shield and colours of my noble ancestor, Salazar Slytherin, will suffice for everyone, won't they, Neville Longbottom?
- Voldemort to Neville in The Flaw in the Plan, Deathly Hallows

No houses? Really? Just ignore Voldemort's vision of a whole school of Slytherins and think about the flaw Dumbledore pointed out. Every wizard or witch is sorted into one of the four houses at the age of eleven and that spends most of next seven years with friends from the same house. Griffindors don't hang out with Slytherins. But what if they would? Maybe if Slytherins were exposed to Muggle-borns, they would be forced to exept that they are not worthless mudbloods.
And of course, seven years is a lot of time, especially for young people, and his or her character can change quite a lot, but others' perception of you is partialy based on which house you are in. Pettygrew is a noturios example. So far I haven't heard a satisfactory explanation on why he was sorted in Griffindor.

Albus Severus, [...] you were named for two headmasters of Hogwarts. One of them was a Slytherin and he was probably the bravest man I ever knew.
- Harry to Albus Severus in Nineeteen Years Later, Deathly Hallows

Just what exactly is the point of Slytherin than? Or houses in general? Besides sterotyping and serving as a filler, none.
When Voldemort wanted to change Hogwarts to Slytherin I of course became outraged. Why? It's not like any he changed any student's personality, just the school's crest.But after that, I pretty much decided that sorting was more or less a bad idea that took the whole everyone-should-get-a-chance feel that the rest of the series had and the fact that the soritng still exists in the next generation was really a let-down.