The first thing I would say about this game is it isn't as good as the first one. In fact, from my experiences so far (I am almost at the end of the third) it is the weakest of the three. That said it is still a great game.
If anyone who hasn't heard of Aang and friends before, no worries, you can still enjoy this game as I hadn't a clue.
This game focuses more on the fire nation rather than Lian and the machines, and Aang learning Earthbending. The levels are fairly linear, with only one type of collectable (scrolls, 6 on each level). Each level you have a choice of 2 players you can swap between, as well as the main people, you also get to play as Jet, Zuko and Uncle Iroh, and Toph has replced Haru as the games Earthbender. Also some levels are playable as Momo.
There are certian areas which require you to fly Appa, these are the low points. While it's good that the developers have tried to add something new, it hasn't quite worked. These are replaybale, as well as an extra one you can only have if you have a Legend Of Aang save game on the same memory card. Other extras include boss battles, gallery (opened by getting the scrolls) and cut scenes. You can also replay levels to get any scrolls you miss.
The journal has stayed and any experince gained from completing objectives and defeating enemies goes to unlocking new moves...which are never used. You can get through the entire game just hitting the same attack button, or the ranged attack if you want to mix it up a bit.
Despite the above, it is a good game and worth getting. I agree with other reviews though that it is very short. Took me about 3 days to get everything.
If you like platformers or the tv show it will be a good buy, otherwise there are many better games out there.