In a nutshell, Lucas wanted to do the Crystal Skulls for the 4th movie since the early 90s, but Spielberg was keen on that idea. After numerous adjustments, revisions, and discussions over the years, Spielberg started to like the story that was developing around the skulls and fully embraced it and really began to enjoy where the 4th film was going.
I don't know, I read pretty much every interview out there, and most of Spielberg's enthusiasm for the movie seemed to be about the actors, and how much fun he had working with Harrison again.
I can't remember hearing him ever rave about the story they were telling-- something which really stuck out to me, because usually he loves talking about the stories of his movies, and the meaning behind them.