The greatest obstacle to a open, free-thinking society in the U.S. is Evangelical culture as it currently is. Perhaps I'm over-privileging my past by making such a claim (having formerly been an Evangelical myself), but no other significantly populated group within Christianity reflects the same degree of homogeneity as the Evangelicals. Sure, there are growing numbers of moderate and liberal Evangelicals (see here and here), but the vast majority of Evangelicals remain both socially and politically conservative (and even "liberals" tend to be socially conservative). This homogeneity actually builds confidence in their own assertions of truth and rightness, and leads many to ignore voices that deviate from what they are so certain is true.
(More after the jump...)
What's more, not only do Evangelicals as a group tend to represent a closed-minded conservatism (see "fundamentalism"), but they are also aggressive recruiters for their cause. It's no coincidence that Evangelicals engage actively in proselytism, utilizing both pseudo-logic and fear/shame tactics to persuade others to sign up on their side. That is, they want more and more people to think like they do, and will go the distance to recruit such people.
For those of us who want to see our society become a more inclusive, discourse-based culture based on egalitarianism and protecting civil liberties for all, it cannot be stressed how important it is to understand this movement, and to get a feel for it on the inside. I don't encourage this to "destroy" Evangelicalism (were that even possible), but such understanding will, I think, help non-Evangelicals and non-Christians break through our own cloud of ignorance and prejudice.
That's why I would direct any of my readers who aren't familiar with American Evangelical culture to head on over to the Stuff Christian Culture Likes blog, written by Stephanie Drury (herself a practicing Christian). Not only will it give you a sharp-eyed look at things from within, but it is just damn funny (especially if you've had first-hand experience within Evangelicalism). I've followed Stephy's blog for nearly two years now, and I consistently find her observations to be as hilarious as they are true. (You might also consider checking out the Stuff Christians Like blog, which tends to pull punches in criticizing Christian culture, IMO.)
At any rate, I only wish I could be as funny in my truth-telling as either of these blogs are. Maybe someday I'll try.