|They answered some questions and left some of the bigger ones open to interpretation. Jack was the focus of this series from beginning to end. The mystery of the island itself is the backdrop of the story. It's clear that the island has been around since maybe the beginning of time. The grand details of each and every question are not necessarily relevant to Jack's storyline. The numbers, the whispers, Adam & Eve, the Others, where Jacob & MIB came from...all these were answered. Maybe not in minute detail, but with enough information to show how they fit into the overall story arc.|
The Mystery of the island was brought to the forefront with the continuous introductions of even new mysteries relating And if many of these plot points weren't relevant then why even bring them up. The only reason you bring up something in a story is if it has relevance.
They answered those questions so vaguely it only led to more questions.