The most shocking thing to me is that our (the US) security agencies seemed to be completely unaware that anything was being planned. No reports of chatter. No outwardly visible concern. Even the President was briefed that ISIS was "contained" and "under control," and he reported as much on national television days before the attack.
This begs the question of where our intelligence agencies are focusing their efforts. Are they really scouring the world for terrorist activity, or are they too busy spying on their own citizens?
We live in dark and scary times when my government knows everyone I call or email, and when, and records all of that communication, but they can't catch wind of a major terrorist attack in its planning stages.