If I write, 'I had a blast at a party last night' (a really simplistic example, but you get the point) or 'Check out this story about this guy who wanted to kill Obama..he was crazy!',does that do anything, or are those keywords ignored?
What's considered a false positive, and what's a true positive? The algorithms do the filtering, but the humans write the algorithms. What's the error rate?
I think I am most afraid of what I don't know about how this works.
Personally, I fear writing certain things with a username that's easily associated to my real name by the public at large, not the government with their clever tools. That's because I know the real and not-so-improbable consequences of doing so[0]: losing my job and getting blacklisted from the entire industry I've spent most my life trying to improve in and contribute to. It can be economic death. The threat of the mob is much realer to me than the threat of the government.
[0] https://handleshaus.wordpress.com/2013/12/26/bullied-and-bad...
[1] http://en.wikipedia.org/wiki/Moxie_Marlinspike#Traveling