Technically, DICE hasn't yet released the "official" Battlefield 4 Battlelog trailer but thanks to a leak, we've gotten an early look at its features. One of the more notable additions is the "Missions" feature.
Missions are small challenges that you can create for you and your friends. These are sort of mini-competitions you can hold among your friends. An example provided in the trailer involved setting up a mission to see who the best tank destroyer is. While competing, Battlelog will notify you if one of your friends takes the lead in the Mission.
While we'll wait to post the full trailer until DICE releases it, the majority of the system's new features seem to revolve around the secondary screen of your mobile device -- be it smartphone or tablet.