I believe you said it all :
-Yeah difficulty curve is a bit harsh, I will certainly fill some gaps between levels, LVL2 is way too hard. And maybe shorten the size of the levels ,like in Super Meat boy, with a "unique" concept for each levels.
-Yep, I'm biased as the developer and probably the most skilled person on this game, it's hard to make a proper difficulty without the help of some play-testers other than me.
-About the tutorial, that was exactly what I had think of when I play your game, "Just show, don't tell"
-I learned that it's really difficult to make a good platformer, between camera, acceleration, speed, jumps, momentum, air control, obstacles, so much variables that needed to be perfectly tweaked, mainly when it's a die & retry.
LVL5 was the last one, so we can say that you basically finished game, Thanks for playing and for your feedback, that was really helpful, I appreciate it.