One of my new favorite bits might be this passage from the beginning of book two:
And yet, will we ever come to an end of discussion and talk if we think we must always reply to replies? For replies come from those who either cannot understand what is said to them, or are so stubborn and contentious that they refuse to give in even if they do understand.
I immediately thought of the comment sections on various sites and the trolls who dwell there. (Naturally, I did not think of the First Things blogs, where all of our commenters are polite and insightful.)
Augustine shows us that even though technology changes, human nature remains the same. There have always been trolls among us, and there always will be.