If you haven't already, be sure to watch the clips in the MTV story where Lucas was interviewed at the AFI 40th Anniversary celebration.
It's funny to see the MTV headline: George Lucas Promises 'Crystal Skull' Will Be As Good As First Indiana Jones Flick
He said nothing of the kind. He was rating the object of Indy's pursuit and ranked the Crystal Skull "up there" with the Ark. It sounds like he is rating it more on its ability to drive the plot, rather than on some fundamental level.