I would argue that there are at least two factors that made Turner's rebellion more frightening than others.
First, it was the bloodiest of all the slave rebellions in the United States. Turner's rebellion ended with more than 55 white people being killed. The "success" of this rebellion made it scarier than those that killed fewer whites.
Second, and perhaps more important, was the timing of Turner's rebellion. It came at a time when abolitionist sentiment had been rising in the North and there had even been publications advocating violent slave rebellions. Because of this, Southerners feared that this rebellion might not be an isolated thing. Instead, it might be something that would become more common as abolitionists stirred the slaves up and encouraged them to rebel. In this way, it seemed more like a threat to the very existence of slavery than other slave rebellions had.