Obviously, the riots, car burning, head chopping... the kind of stuff you'd see on the news. But more importantly in the long run, it simply doesn't work, much like many of the immigrants who come here. See Malmö for example[1] where "the majority of foreign born working age adults do not work." That, and every other indicator makes for a bleak picture of immigration.

[1] http://super-economy.blogspot.com/2013/05/malmo-case-study-i...