I know I'm a little late to the discussion, but I think that the reason that the Slytherins are often portrayed as being nasty is that it's primarily full of rich kids who have been taught that they are better than others because of their heritage and wealth.  Due to their upbringing, these kids would probably also have a lot of self-confidence, which makes them more likely to become bullies, especially if they consider those of less pure blood status to be inferior to them.  Additionally, the traits that the Slytherins idealize, cunning and ambition, more easily support malevolent intentions than say bravery or good work ethic, which made them easy targets for Voldemort's recruitment plan.

While it certainly seems that the Slytherin house was set up to be an antagonist in the early books, it is important to remember that, as Prints said, that the perspective is of a young boy and his friends who are constantly being antagonized by a group of Slytherins.  Towards the end of the series, however, we see that the Slytherins are not primarily evil, especially considering only one Slytherin suggests giving Harry to Voldemort and they join in the Battle for Hogwarts with reinforcements from Hogsmeade.

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Having no knowledge of the Houses beyond general traits as stated by the Sorting Hat (a fairly neutral source), Dani has no preconceived notions as to the character of people in other Houses or even her own.  In fact, despite being a tad upset with Wade and those he had been fighting with for getting her in trouble (she, Pearleane, and Peter had done nothing), she doesn't harbor any resentment towards the other members of the team or the House.  She actually has a fairly good opinion of Tim at least for being rational and listening to her suggestion that he try and stop Wade from escalating things.

In other news, I will try and get a post up soon. :D