1. Aang has hair now. From what I see, he has been unconscious since the finale of season 2 and no one felt the need to shave his head. Since his hairline doesn't cover the rest of his arrow, he'll be wearing a headband/belt over it while in the fire nation.
2. Aang decides that he has to fight the Fire Lord alone. I don't know how solid of a decision this is because we later see scenes of Toph and Katara fighting with him.
3. Everyone has new clothes this season. Thanks to a person recreating them, we can now get a general idea for the gaangs outfits. http://community.livejournal.com/avatar_fans/1721262.html#cutid1
4. Zuko and Mai hook up (if only temporarily) this season. If you watch the trailer the two kiss very deeply. I still stand by my beliefs that this pairing is nothing to worry about. Zuko is a good guy despite what side he's on at the beginning of this season. Mai is a bad guy and doesn't show promise of changing. Unless Mike and Bryan decide to go against all of Zuko's character development, Maiko with not be the final pairing.
(*Side Note* I like Mai. She reminds me of a slightly less passive Hanajima from fruits basket. She a cool character and a good balance for Azula's group, what with Ty Lee and all. I do not however like the Maiko pairing. I don't see why so many people support this pairing when there's no why it could logically become canon. It's like claiming that Jetara is going to still be canon despite the fact that Jet died.)
5. Iroh is still alive. He's a prisoner and in terrible shape, but he's still alive. The only scene we saw of him was Zuko saying that he was foolish for helping the avatar instead of letting him be captured. Yes we all want to smack Zuko, you are not alone.
6. There is a beach epsiode. I don't know if the gaang is in it, but Azula, Mai, and Zuko (as well as maybe Ty Lee) are definitely in it. Azula's outfit looks kinda princess'y with a skirt thing and she has hair hair down in the pic. I haven't seen Zuko's or Mai's but others described Zuko as a cabana boy and Mai as wearing a hat so the sun doesn't get to her. (She's more and more like Hanajima everytime I see her. I have heard anything about Ty Lee for this episode as of yet.
7. Zuko leaves the fire nation. The pre-view shows Zuko being unsure about life in the fire nation at least once, and goes on to show him gathering his swords, putting on a hood, (much like Ursa did) and leaving. This only suggests that he's not doing something that Azula and the others would be okay with. If he was just going out for some training, he could just tell them and go in daylight, no one would care. Since he's going on night and obviously trying to be inconspicuous only furthers the proof that he's going against the fire nation and one step closer to becoming a good guy. More than likely he's also going to search for his Mom or information about what happened to her.
This is all I think of right now, but I might remember something later and post about it.