All courts agree that "true threats" are not protected by the First Amendment but that "rhetorical hyperbole" is protected (e.g., a poster on FR who says "Obama is a traitor and should be hanged.")
The issue before SCOTUS--and one that has split the lower courts-- is whether the jury should be instructed that it had to find that the defendant intended to threaten the victim, or whether it is enough that the victim reasonably believed that she has been threatened. (In other words, in deciding whether something is a "true threat" or "rhetorical hyperbole," do you look at it subjectively and from the defendant's perspective, or objectively and from the victim's perspective.)
Here, the victims all testified that they felt threatened, but the defendant claimed he was just "blowing off steam." (The jury probably wouldn't have believed him anyway, but they were instructed that his intent didn't matter.)
I think intent matters and words matter. That he wrote and made threats, explicitly, is very much different than a person suggesting someone should be hanged. The intent was obviously and he rightfully spent three years in jail. The slippery slope is when we start making excuses and allowing behavior that is somehow excusable because it is on the Internet.