The claim to greatness comes from the length, penetration and scope of John Paul's pontificate, as he defeated communism, traveled the globe as a celebrity ecumenist, and left his mark on every stone in the edifice of the post-conciliar Church.
But there is a paradox that would probably prevent the epithet from taking hold. We are convinced, aren't we, that Pope Benedict was meant by John Paul to be his successor. Then John Paul's legacy is forever linked to Benedict's. If Pope Benedict succeeds in his mission to save the Western Civilization, then he, and not John Paul will be remembered as Great, even though John Paul's hesitation in confronting the faithless bishops will be thereby vindicated. And if Benedict fails, then John Paul's lasting legacy will be his indecision.
And that is why these contrived, on-the-spot acclamations are wrong. Chesterton has spoken of Tradition as the democracy of the dead. There's a democracy of the unborn as well: we have no right to pre-empt the judgments of history.