From the Official Website:
Super Street Fighter IV Arcade Edition features a roster of 39 characters such as Ryu, Chun-li, Juri, El Fuerte, and Rufus, while adding 4 new characters to the roster including classic combatants from previous Street Fighter games like Yun and Yang as well as the all-new Oni. Beyond characters, Super Street Fighter IV Arcade Edition adds all-new balancing as well as an enhanced Replay Channel.
1- New Playable Characters: Four new challengers enter the ring: Yun, Yang, Oni and Evil Ryu.
2- Updated Character Balancing: Features all new character balancing, further refining gameplay.
3- Updated Replay Channel: Replay Follower: Be able to follow up to 5 separate player replays making it easier to find your favorite players' latest replay. My Channel Advanced: Allows players to distribute their replay data to up to 50 players. Elite Channel: Watch replay data from elite players who have a rating of 3000PP or more.
4- All playable SSFIV characters will be unlocked out of the box, including Yun & Yang, Oni, and Evil Ryu.