It's quite the other way around: threats must be very specific in order to not be protected speech in the United States. Saying 'let's burn this mother down' during a protest is protected speech, for instance. In general, the threat must be specific and imminent, such as 'let's go to that building right there and do $violence right now!'

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.