Nothing! There are those who point out that the door latch is missing, but if you'll look again you'll see that Christ is standing in front of the latch! The door hinges are on the right. That would put the latch on the left, and Jesus is standing in front of it.
Why, you ask, would anyone say that the door latch is missing? They have a point to prove in theology. They want to prove free will. They want to prove that Christ won't (read "can't") come in until we "let Him come in."
If that is true, then I'm afraid all of humankind is lost, folks. If Jesus can't come in until we "let Him," then He'll have to stand out there for a long time.
For more on this subject, read my article "God Can't Do a Thing About It?"