You skip all outdoors stuff and a couple of the rooms of the main queue, but in my opinion it’s nothing too important. The preshows are main priority and Express does not skip them.
I still think it’s worth waiting in standby at least once to see the whole manor, but it doesn’t have to be your first ride as long as you don’t skip preshows.
EDIT: All the Express lines are good and don’t miss anything important for the whole park as far as I’m concerned.