According to the trailer, the new DLC adds the arcade-only Yun, Yang, Evil Ryu and Oni characters, along with the changes to character balance made for the Japanese arcade release -- which contradicts a previous statement made by producer Yoshinori Ono. Those who download this add-on can still play against non-DLC Super Street Fighter IV players.
The update also enhances the "Replay Channel" with new features that allow you to send replay data to friends, "follow" certain people and watch their new replays, or watch only fights between people with 3000 PP or more.
Update: The original video we embedded has been pulled by Capcom. We've embedded another one, but, well, watch it while you can.