At least with the people I know, the biggest problem is rabid, blind devotion to the Church of Liberalism. Or Progressivism, Secularism, whatever you want to call it. My conservative and practicing Catholic friends tend to stay level-headed and calm while discussing things. They recognize the fact that other people have differing opinions and are entitled to them, no matter how wrong or harebrained they may be. My liberal friends are the flip-side of the Westboro Baptists. Not that they’d admit that because they’re 100% correct in each and every situation, but several in particular instantly resort to throwing around the f-bombs and calling you a racist and/or Nazi if you even hint at disagreement with them. Not only is even a whiff of dissent evilly wrong, but you need to be viciously and personally attacked for even thinking that you’re entitled to have beliefs that don’t line up with theirs (and, let’s face it, it’s far easier to do that than to try to actually argue against an opposing viewpoint).
One example that always springs to mind is several years ago when I disagreed with something my one cousin said about some political issue. I can’t remember if it was the environment, gun control, George Bush or whatever; it wasn’t a huge issue but they’d made a false statement and I countered it with something I’d happened to read a day earlier. Instead of addressing what I’d said, she turned to my wife and said she’d expect that from “someone like him,” then told her that I’d cheated on her at a party years earlier. It wasn’t true and the situation was quickly defused, but it got me and the few other non-liberals in the family to quit discussing anything political with her to avoid having her make claims like that around our kids. At the time I didn’t consider cutting ties with my family as an option, but she’s one of the primary reasons we no longer see that side of my extended family.