I don't think Reformed theology or any other tradition really bears on this issue so much as simple hermeneutics. Jesus spoke using many traits of ordinary language , and forcing an interpretation on the passage that does not take into account the ordinary ways that language is used and people communicate ideas only leaves people with twisted conclusions.
Here Jesus is not making a point about John so much as he is making a point about the significance of being included in the Kingdom of heaven. There is a comparison being made about relative significance, but this has nothing to do with passing judgement on John as being in or out of the kingdom. In fact we have every reason to believe that he was himself included in it.
Basically, your asking the wrong question. It's not a matter of what John the Baptist lacked or that made the other disciples great. In fact the point of the passage is exactly the opposite of that: the success or failure, greatness or smallness of our lives from an earthly view has exactly no bearing on our status in the Kingdom.
The point about John the Baptist was simply that although he played a very special role history -- a role itself foretold as one who would prepare the way at the coming of the Messiah -- this did not make him special in the Kingdom because that placement is dependent on the work of the Son of Man, not that of John. What greater honor could a man have than to be the immediate herald of the greatest event in all of history? And yet that honor and distinction is shown as insignificant compared to the honor that we are all given as believers grafted into God's family being made co-inheritors with Christ in His kingdom.
All this verse shows us is that any earthly rankings in honor are utterly irrelevant when it comes to our membership in the Kingdom.