Continuity mistake: When Rey rescues BB-8 from the scavenger Teedo, the top part of Rey's outfit is crisscrossed with her right side over left, but when she shushes BB-8 it's crisscrossed with her left side over the right. Then it's back to the original way. (00:15:25)
Trivia: A model of the tumbler from The Dark Knight trilogy is spray painted on the white hull of the Millennium Falcon, and mounted to its underside. This is because there was a friendly competition between JJ Abrams and Zack Snyder to see who could one-up the other by putting Batman/Superman references in Star Wars and vice versa.
Chosen answer: The actors playing Rey, Finn, Poe, and Kylo Ren were mostly unknown, while Mark Hamill is famous worldwide as both a screen actor and voice actor. The billing order was likely based on their fame and then on their new roles. It's not a union thing, minimum wage in the Screen Actor's Guild is far lower than what they were paid. It's a combination of bigger star, better agent, and the future of the series.
Greg Dwyer