Hmm, I'm pretty liberal but there's definitely some pretty large "slum" or "ghetto" areas outside certain major cities in Europe. Generally they're predominately unassimilated refugees as far as I can tell. There doesn't seem to be much attempt at integration really. I am unsure about whether the police will go to them or whether or not they are more or less violent than American cities.

There are certain things the US does well, I think assimilation of immigrants is a really strong point for our culture here. Maybe it's because our culture has never really been much more than a gigantic mixing pot and is a lot younger. And maybe it's cause the population is so much larger and discordant than any single European nation.