I absolutely loved this game; the music, the environments, the art style, the hidden stuff, the languages they made up; everything. Will be there on Day One for the sequel.
One thing I thought was under-used in the original was the lower halves of the cities. It felt like there could have been a lot more exploration and action down there but that they didn't want to pull players away from the high-flying gravity-manipulation core of the game. I would rather have seen them mix things up not by taking away Kat's powers, as they did for a brief period, but by having a claustrophobic indoor-like portion set in some of those lower levels. You only really go there to pick up gems, and even then you don't really do much down there.
Please tell me that "Fall Again" is a double entendre and that it's coming out in the Fall of 2013.