Rose brought up a good point about Jack getting sick. WHY did he get sick, on that island, of all places?
There are a couple of theories that are floating around:
1) The island is punishing him for some reason. It wants to prevent him from leaving.
2) There’s a theory going around that it wasn’t appendicitis at all. Juliet did the operation to take out a device that Jack had implanted in him. A device similar to what made Claire ill. It does fit: Juliet was shaving him in an area where the appendix wasn’t, and Jack was suspicious. I expect her insistence on getting him knocked out was because of this.
I think theory #2 is right, and it brings up an even bigger questions: Is Juliet hiding this from Jack because she’s, even now, working for Ben? Was that done to try and insure that Jack would stay on the island?