For me, the worst thing is that Reddit users, judging by their posts, have a tendency to assume that they are more intelligent, more rational, and more open-minded than the average person - when really they're just not. Watson's dual example of skepticism brings the point home, really - many redditors (especially the super-atheist) variety assert a greater level of supposed skepticism, but really they're skeptical about some things (God, Kony) and entirely driven by their own biases with regard to others (rape victimhood). I hate it when people assert that they are open-minded, because they almost never are. And anyway, I think "open-mindedness" can't be regarded as a static quality that people possess in a more or lesser degree but a mental practice - sometimes people think in a mind-opening way, and often they keep their mind closed.
Its because they're young. Youth is not a time when you are rational; trust me I've been through that stage. When you lack a sense of failure, when you're in the environment you're in as a kid, everything looks crystal clear. Most people realize the world isn't once they leave high school and head off to other things. They find that people are as varied as they are, that not every republican is an asshole and not every christian hates your guts. They also, for better or for worse, develop their own set of stereotypes to use daybyday. Open-mindedness is something that can be relatively static and it should be something people strive for. The problem is when people use it for posturing. Being open minded about new experiences helps to enrich your life and creates some of your best memories. That's not something to show off, its something to live. Maybe I'm just being closed minded though.