He is not a prophet like any of the prophets...he must be John the Baptist come back to life. Herod heard of Jesus' fame, and shivers ran down his spine. What he was hearing about Jesus reminded him of John the Baptist, his old nemesis. He saw Jesus as more than the earlier prophets, and likened him to John the Baptist. What was it that Jesus was doing that made him to see John the Baptist in him (Jesus)?
The Baptist came preaching the imminence of the Kingdom of God and called people to repentance (cf. Matthew 3:1-2). His preaching had effect on his hearers, for many flocked the river Jordan to be baptized by him and to confess their sins (cf. Matthew 3:5-6). This effect must have been brought by John's style of preaching. The Baptist neither preached a blanket repentance of sins nor a general action plan. He pointed out each individual's particular sin and prescribed a matching penance (cf. Luke 3:1--14). This is how the Baptist got to cross paths with Herod. 
When Jesus appears on the scene, he picks up from where the Baptist had left. He too comes preaching the imminence of the kingdom and calling people to repentance for the forgiveness of sins (cf. Mark 1:14-15). Jesus' ministry also touches the people on a personal basis, and just like the Baptist, he too gathers people around him. And it is herein where the concern of Herod lies. He has never met Jesus, but he has heard of the crowds that throng the places where Jesus pitches his tent. Jesus has not attacked him personally, but he is getting scared that these crowds might turn out to be revolutionary. The Baptist wanted to rid him of his wife; Jesus wants to take the kingdom away from him.