Questions left unaswered

Jimmy Kimmel asked, where did the polar bears come from? What about the Smoke Monster?
Did we ever get a definitive answer on that?

I also came up with the following:
Did they ever say why women couldn't give birth on the island? And why men became more ... productive? Did it have to do with the electromagnetism stuff? How about Locke and Rose being healed?

Why was Walt special? Where did his gifts come from? What about Hurley? How come Hurley was the only one who could see ghosts? We know where Miles' gift came from. He was born on the island.