What is the difference in response between Zechariah and Mary?

Zechariah did not receive a rebuke but a sign appropriate to the calling of John.

   1Ki 17:1 And Elijah the Tishbite, who was of the inhabitants of Gilead, said unto Ahab, As the LORD God of Israel liveth, before whom I stand, there shall not be dew nor rain these years, but according to my word.

Elijah was introduced to Israel by calling for a drought. There would be no water until he speaks.

John has been ordained to fulfill the prophecy that one like Elijah would prepare the way. Historically, God has been silent for four hundred years. He has not spoken to Israel through prophets, kings, priests, or judges. There has been a drought of the word of God until John.

What more appropriate sign can there be that Zechariah cannot speak until John? It was a blessed sign indeed, not a curse.

On the more esoteric level, Jesus is the Word of God and the Living Water. As the forerunner to Christ: There is no Christ until John speaks. It is he who proclaims "Behold, the Lamb of God!" after which Jesus begins his ministry.