When Jack meets up with Desmond at the stadium where they are running the stairs, they talk about Sarah and the fact that Jack promised he would fix her but he didn't. Desmond asked him a couple of times "what if you did?" and Jack kept saying he didn't.
Of course we know that the operation was successful. Did Desmond "heal" Sarah or did he just see into the future? I watched it with my husband who is a first time watcher and he definitely felt Desmond healed her.
I think he is a huge piece of this story. I have always enjoyed the character, but never really given a lot of thought to his importance. How do you guys think he figures in or are you clueless?