Here's one I just came upon, from one of my favorite authors, G.K. Chesterton:
"The whole curse of the last century has been what is called the Swing of the Pendulum; that is, the idea that Man must go alternately from one extreme to the other. It is a shameful and even shocking fancy; it is the denial of the whole dignity of the mankind. When Man is alive he stands still. It is only when he is dead that he swings." - "The New House" Alarms and Discursions
This quote so nicely encapsulates one of my biggest frustrations...extremism.
Having grown up among very strict fundamentalists, you can imagine I was exposed to my fair share of extreme thought and practice. As I grew older the extremism bothered me, and yet not enough to swing to the opposite end of the pendulum and throw out the baby with the bathwater, all the good I did learn I acknowledged and kept.
So it frustrates me when I see huge leaps from one extreme to another, usually in the form of a complete rejection of something without carefully considering if it is all worth rejecting or not. Then you end up with people on both extremes calling those in the middle "extremists" simply because they don't agree 100% with their ideas, and that's how I often feel, stuck in the middle!