Fan service = ride on the nostalgia of the original, bring back some original characters, have them wink at the audience and be charming, then often die to "make room" for the "new blood"

Mystery box chasing = there's some object that the good guys and bad guys both strive for, it's the maguffin, there's no underlying theme in the movie other than this mystery box chasing, and the mystery box is ultimately hollow and meaningless, he talks about it here: https://www.ted.com/talks/j_j_abrams_mystery_box?language=en

(http://www.autoadmit.com/thread.php?thread_id=3073734&forum_id=2Elisa#29407126)