As I sit in my basement and type away, I think I don't have anything to worry about. I don't say anything (very) inflamatory, much less seditious, so the guys from the alphabet soup jungle (FBI, NSA, CIA, etc.) wouldn't be interested in anything I say. But that's not the problem.
The problem is not that the government could use incriminating evidence to track down terrorists. Most everybody is fine with that.
The problem is that a bad apple somewhere in the government could use some of this mass of innocently collected information to target someone who has done nothing wrong, but for some reason has attracted this bad apple's attention, and either through vindictiveness, outrage, or simply boredom, has decided to bring the power of the federal government to bear on this one individual.
The power of the government is tremendous. It needs to be carefully controlled. Giving the government unfettered access to private communications is like giving them a surfeit of ammunition, and as every gunner knows, when you have more ammo than you need, you tend to shoot more. I mean, that's what ammo's for, isn't it?