The Lost part of 20/20 was only 30 minutes long. They didn't give much info, other than that we, the audience, apparently know more than what the characters know re the backstories, which makes a difference in how the show is viewed. Apparently no one knows what the monster is.
Terry O'Quinn was asked what he was looking at in the scene where he sees the monster, and he was told that it was "The most beautiful thing he had ever seen" or something like that.
They talked to Boone.

He seemed so upset about the death and said it was really hard to shoot the death scene. He said he has gotten close to Jack so seening Jack have to do the death scene also made it hard for him.
This is Kate's first speaking gig as an actress. As the female lead, I think she was one of the last to be hired.
Sawyer was about to give up and quit acting when he got this gig.
Hurley is apparently america's favorite, according to 20/20.
Sun originally auditioned for Kate's part but didn't get it. They liked her so much that they created the characters of Sun and Jin just for her.
They apparently were really excited about having Micheal act in Lost. They loved his previous work.
They showed some parallells with their influences. Pregnant Kate = the lady in Rosemary's baby (Can't remember who). The hand coming out of the ground when Jack/Charlie dig out of the cave = the end of Carrie when the hand comes out of the ground. Their biggest influence is Twilight Zone. They pretty much said that Lost ISN'T purgatory and that they thought any sort of "This was all a dream" ending would be a big let down.
Walt doesn't want anyone in the cast to die.
I'm sure I missed some things. In a nutshell, nothing new re the plot was revealed.