I agree with Typhoid. The questions and the mysteries were just a way to propel the story arcs of the characters ... when we didn't know much of the background of the characters, the questions were the focal point of the show, but as we began to relate to and empathize with the characters, they became the focal point.
LOST, at its core, was a love story, and if you aren't one for love stories then I could see some cause for displeasure. However, probably should have seen it coming - it was always about love.
Also, the island was certainly not purgatory. Perhaps the sideways universe was purgatory, or it was a concocted universe by the castaways as a way to find one another before moving on. Jack was the last to *find* the church, but he was not the last of the castaways to die. I would assume Hurley and Ben lived to a ripe old age. It was interesting that Ben did not feel comfortable entering the church: perhaps he still felt as though he had not properly redeemed himself? Or perhaps he was waiting for Alex to move on? I quite like the latter option.
Edit: If you watch
this TED talk by J.J. Abrams (entitled Mystery Box), the creator of LOST, you might get a better idea as to why there wasn't a need for concrete answers. The talk is captivating and engaging, but Abrams fails to reveal just quite what is truly inside the mystery box, but does it matter? I don't think so. At a point, the answer becomes irrelevant, and the journey and struggle is far more important ... kind of like life.