This game is amazing and I will entail my full opinion on it below:
Awesome (as in, it feels as it should or quite close):
-The idea of this game reminds of a lot of games that I enjoyed/enjoy
-Upgrades (they definitely make the game experience feel better and better)
-Slowly putting the player through more and more challenges
-"Secret" areas
-Boss fights (I only experienced the first one, but I am sure there are more)
-Music / Sound
Not so Awesome:
-Lack of enough checkpoints (I am not saying to put a whole lot more - and to be clear, I am talking about the normal ones and not the portal ones - but this is one of the reasons that I, as well as other players I am sure, eventually gave up on pressing onwards)
-Flying enemies that come towards you -> the idea itself is good, however, either due to the level design (having a lot of crumbling platforms instead of normal ones while dealing with them is a bit annoying and so is falling for a long time and having to wait for them to reach you) OR due to the fact that you cannot aim easily enough (with the mouse for example) to take them out and most players will wait for them to be on the same horizontal plane as them before trying to actually destroy them
-Level Design - now, this is not comparable to the two above, however, having more "easily reachable" platforms/areas would definitely help the feel of the game instead of making the game feel like you are always "barely reaching" everything around you - I mean, you have magical wings and you should feel like you are actually reaching stuff easier due to your "double jump" instead of making this a necessity - this should feel like a privilege to have
Lastly, the only thing that I want to say is that I wish I had more time to play your game xD
Highly enjoyable!