It made the world weird - especially politics. I still attribute the extreme polarization that we see today to the aftermath of 9/11.
Don't get me wrong, I know people had strong opinions before 2001, but it didn't seem like political party was as significant a part of the average person's identity like it is now.
I think there's a pretty strong case that the polarization started before 9/11 - Rush Limbaugh, talk radio conservatism, and the race for evangelicals had been making US conservatives more polarized since the 1980s. The attacks might have made it more apparent, but commentators were decrying polarization in the 1990s.