Just finished the game, had a decent amount of length to it and I absolutely adored all of it.
The physics of flying are so smooth and fun. Even though it can be hard to complete certain challenges, I always felt like my failures were my own. It's all very consistent and solid, both on the ground and in the air.
The atmosphere is wonderful as well. Throughout my time playing I always had this sense of wonder, unsure of what I would see next, thinking about what all these strange words and pictures might mean.
I love the visual style and characters too, and the way they're animated is impressive and extremely convincing (and often adorable as well of course). The music is also great, so wild and varied!~
I could really go on and on about everything I liked, instead I'll just say my only disappointment is that I still want more ;P