JOEL - Here are the scriptures where Jesus explains John's mission:

"And as they departed, Jesus began to say unto the multitudes concerning John, What went ye out into the wilderness to see? A reed shaken with the wind? But what went ye out for to see? A man clothed in soft raiment? behold, they that wear soft clothing are in kings’ houses.
But what went ye out for to see? A prophet? yea, I say unto you, and more than a prophet. For this is he, of whom it is written, Behold, I send my messenger before thy face, which shall prepare thy way before thee.
Verily I say unto you, Among them that are born of women there hath not risen a greater than John the Baptist: notwithstanding he that is least in the kingdom of heaven is greater than he.
And from the days of John the Baptist until now the kingdom of heaven suffereth violence, and the violent take it by force.
For all the prophets and the law prophesied until John.
And if ye will receive it, this is Elias, which was for to come.
He that hath ears to hear, let him hear." (Matt 11:7-14)

Besides being the one who baptizes Jesus, as you mentioned, in these verses Jesus explains that John is more than a prophet; he is an Elias, because he is the messenger that all the prophets testified would prepare the way for the Savior (Mal 3:1). And as an Elias he will turn the hearts of the fathers to the children and turn the hearts of the disobedient to the truth.

"And he shall go before him in the spirit and power of Elias, to turn the hearts of the fathers to the children, and the disobedient to the wisdom of the just; to make ready a people prepared for the Lord." (Luke 1:17)

He prepared thousands of people who listened to him and were baptized by him, all being told by him that someone greater was to come. No other prophet had done this before to the degree that John did.

Also, at that time John was the only legal admisistrator of the kingdom of God on earth holding all the keys of power, and as such the Jews were obligated to obey him or be damned according tho their own laws.
According to Jesus, all of these things combined made him "more than a prophet".