I liked that the fourth book finally told us that the defining characteristic of Slytherin was pure-bloodedness and snobbery. It actually makes sense then as opposed to the first three where you kind of assume that the heroes go to Gryffindor, the villains go to Slytherin and the other two houses get sorted by who is dumb or smart.

Reminds me of Chasidic Jews who won't let converts marry into their families or they will make sure to set up their daughters with the "good" families. Apparently Satmir actually take family trees from ten generations back to make sure that there are no converts in that tree (but this is only for the rebbe's family and the other Satmir just inbreed but if they let someone from a non-Satmir or even non-Jewish lineage in, then they guess it will work that way).

I also base this on the fact that among non-wizards, all wizards are considered stupid and scary and unimportant so the prejudice is mutual.