Jesus challenges the crowd on what they believe about John. They had all flocked out to see his famous Jordan River baptisms. Why did they gawk at him? What were they expecting to see? Certainly not a soft-willed man dressed in soft clothing, right? Perhaps at this point someone from the crowd shouted, "A prophet!" So Jesus answers, "Yes, I say to you, and one who is more than a prophet." John was not just any old prophet but the greatest in a long line of prophets, since he was set apart to prepare the way for the Messiah's coming.
Behold, I send my messenger before your face, who will prepare your way before you.
John is the greatest of "among those born of women" (which is just another way of saying among those who are born), and yet the least in the kingdom of heaven is greater than he. I don't think Jesus is saying John is directly inferior to every person who enters the kingdom after him. Rather he is drawing a partition between the old covenant era, of which John's ministry is the climax, and the new covenant era that Jesus has come to inaugurate. The glory of the new covenant so surpasses that of the old covenant that the least member of Jesus' heavenly kingdom participates in something far greater than what even a great man like John the Baptist had.
Why is the new covenant era superior to the old? It is the difference between promise and fulfillment, between shadow and substance, between hope delayed and hope realized. John preached a message of sorrow and repentance, of humbling oneself before the coming wrath of God. After Jesus' death and resurrection the apostles would preach a message of rejoicing and forgiveness, of humbling oneself in light of the riches of God's grace and mercy. Even John, the very forerunner of the Messiah, could see only dimly the glory of the kingdom that Christ would usher in, and the least of us who stand on this side of the cross know a joy and intimacy with God that the Old Testament saints could only hope for from a distance.