The title says it all, and the preorder bonus comes with a special weapon and a free future DLC!
As for a cool little research project, there are some good articles out there talking about the problems arising from buying and selling used games and the effect it's having on the gaming market. Many companies now offer these preorder bonuses just in order to combat the distribution of games this way, and EA is even taking steps to restricting gameplay to those who buy them used.