At first I was like FUCK NO! But after reading it, it looks more like a countinuation of the series instead of a recreation of it. And it would be nice to watch a 2D animated movie in theaters again. You never really see those anymore.
Why are people so surprised about using fire bending to fly? Ozai did that in the ATLA finale. Granted, Sozin's Comet was passing over and his power was at his peak, but who's to say future generations can't get better at physical feats? Just look at what the athletes of the Olympic Games from decades ago did and compare that to what today's athletes can do.
Well there was a whole episode to Bloodbending in ATLA, and there quite a few for Metalbending after Toph discovered it. We had already gotten familiar with them, or at least remember them. They only showed flight with Firebending at least three times (maybe), and all three times there was a lot going on in the show. Azula did it once during the escape from the Boiling Rock, and then Ozai along with Jeong Jeong did it during the season three finale. A few brief moments of flight tend not to stick during all that action. Of course that's saying that memory is somehow tied into to people believing it.
On a side note, Azula also used her bending to propel herself on ground level in the season two finale.