All the same, threatening the judiciary is one of the most anti-social things you can do, and should be strictly policed. I don't know if trawling news comments is an effective way to do that, but I'm glad to know they're making an effort. A few false positives (appropriately handled) is a good sign of sufficiently high sensitivity.
Do you have a source for that beyond "the 1st ammendment"? This feels like the kind of thing that has an exception.
I think "inciting violence" is not covered. Certainly, threats are illegal, not sure how veiled those threats need to be to circumvent that.
They have been mentioned several times in this thread, but you should consult Popehat's writings for in depth breakdowns of what is and is not protected speech.
https://en.wikipedia.org/wiki/Imminent_lawless_action
Pointing at a person during riots and screaming "Kill that muthafucka now" would almost certainly cross the threshold.
Organizing an unofficial Wannsee conference to plan a genocide of $UNDESIRED_GROUP into details would not, even though the entire event would be a hatefest.