Jack Knows and Juliet's Mind Traveled by Logic

I don't think we can even begin to guess why the island is underwater or where home is for the man who resents being called "the monster." Those will be answered eventually.

However, what is obvious after watching the season 6 finale (twice) is that Jack was probably aware everything had been reset when the plane didn't crash.

When he was clenching the side of his chair, he was anticipating a crash. When it didn't, I guess he assumed everything worked. What makes me believe that he knew is his reaction to the characters on the plane. His confusion in seeing Desmond wasn't because he recognized him from their meeting a few years prior, it was actually because he wasn't sure why Desmond was on the plane at all. This is probably where he realizes that things might be different this time around. His encounter with Cindy is also significantly different, like he rehearsed what he was going to say.

Also, a rather odd comment from Sawyer in the remains of the Swan hatch in 2007 was "for once you don't know," when referring to if the bomb went off. This was a subtle hint from the producers. I think it comes down to Jack always being right. Jack thought he should leave the island back in season 3 because he wasn't meant to be there. If Jack had never left with everyone, it would be different for everyone there because his return sparked all of the events we saw at the end of season 5 and, probably, the eventual reset. Although he appears broken now, Jack will eventually be proven right. He will be the hero.

Finally, Juliet knows it worked because her mind traveled sideways. I'm sure we will eventually see her asking someone out for coffee in the "flashsideways" events. Most likely it will be Sawyer.