I was thinking about my answer before I asked myself the question...
I'd say the answer isn't nuclear war because for most of us we wouldn't have time to even worry because it would be over fairly quickly (Or at least I think so based on my limited knowledge on the subject)
MY ANSWER: Controlling how things are perceived in modern society (The Norm)
Now this has always been a problem but why now more than ever?
Just think how fast information is shared and how quickly trends are created. Mostly positive but you can image how a certain trend could get hugely out of hand, spread to the masses and be seen as a norm.
I haven't really thought about my answer too much and it will probably change BUT I would be interested in knwoing what other people think....