For one, they preached different messages. Elijah's was a primarily a message of challenge to the king and the godlessness and apostasy of Israel. John the Baptist did confront the king, but his message was primarily one of preparing the way for Christ.

Elijah was a prophet when the very foundations of Israel were being destroyed by moral decay—the covenant was being subverted. John was a prophet when the Messiah would come to bring a new covenant.

Elijah's message was "Repent, you are apostate." John's was "Repent, for the kingdom of heaven is near." John baptized with repentance in view, Elijah didn't. Elijah rebuked with repentance in view.

I wouldn't say they were "quite opposite of each other." They had similar messages for different audiences at different spiritual junctures, and so their messages diverge off of the same foundations.

We all know the similarities between Elijah and John the Baptist. But what are some of the differences. Like where are they quite opposite of each other?