A lack of reason...
On a slightly different tack that CTBD's thread, I thought a make a few more points about reason.
I tire of those who endlessly argue - not debate, for that implies rationality - without ever listening to the arguments presented by others. These are the ones who persist in hammering topics that their own side has already acknowledged are either dead or conceded to the other side. These exist in every debate, on both sides. While there may be a reason to follow up on topics if there is new evidence, it is illogical to argue something that contradicts the position of other rational members on the same side of a debate.
It seems clear to me that the "Age of Reason" never really hit. It is also clear that reason can obviously ony attain so much in humanity.