I agree the Angel/Satan description is too simplistic. But those were some quick thoughts.
Any theory will have to include the parallel world, and right now, I have no theories on that.
Some things of note in the parallel universe:
- The plane doesn't crash (duh).
- The island is at the bottom of the sea (duh), and presumably now destroyed.
- Shannon (the tall blonde that Sayid fell in love with early on) was not on the plane, because her stepbrother, Boone couldn't convince her to leave her boyfriend in Australia. I don't remember the backstory here -- can it be explained by the fact the island is destroyed?
- Desmond is now on the plane, presumably because the island is destroyed and so he never went to the island and never caused the plane to crash. He also (perhaps) disappears from the plane before landing.
- Jack seems to recognize Desmond. Can that be explained normally? Didn't they meet before?
- Locke is still paralyzed and presumably lied about going on the walkabout.
- Hugo is now the luckiest man in the world -- presumably because he didn't use the numbers from the island to win the lotto.
- Jack had blood on his neck -- from what?
- Charlie tried to commit suicide on the plane. I don't remember how that is different.
- Jack's dad Christian and his coffin are now missing from the plane.
- Locke's bag of knives is missing from the plane.
Anything else?