I'm gonna lay out my own pet theory about the season finale before I forget about it.
Jack and the Losties weren't just rescued back to the mainland - they were rescued and brought back in time.
A few things: When the hatch imploded (or whatever it did), Desmond went back in time and met someone else who also received visions. Assume that this actually occurred in a parallel timeline. It could be that certain members of Dharma (or some other faction) are able to insert themselves into other timelines - that they could use this to learn hard-to-find information or alter the course of events.
Regardless of this, back to my main conceit. Assume that the Losties on the beach were left behind.
If the Losties were rescued and brought back in time, this would explain:
-Why Jack spoke about his father as if his father were still alive. In the new timeline, Jack was able to stop his father's death.
-Kate's appearance and actions. In the new timeline she is still married to Callis. She is pissed at Jack both because of her feelings for him and because Sawyer was left behind.
-Why Oceanic Air is still in business. In the main timeline, the airline went under because of the crash. In the new timeline it is still in operation.
-Why nobody recognizes Jack as one of the survivors of Flight 815. Because in the new timeline it didn't crash. Maybe the flight never even happened. If the Losties were able to convincingly warn Oceanic about the upcoming crash, this might explain the 'gold passes' that allow the Losties to fly anywhere for no charge.
-Why Jack wants to get back - he couldn't save everyone.
I was reminded of two different works: the film Millenium and the novel Timequake.
There are still a ton of questions, of course. I'm probably completely wrong. But it's fun to piece things together.