Hi, thank you very much for taking the time to play through and writing this feedback (sorry about the bats, must have been frustrating).
Regarding the art style: all the BEAUTIFUL background and grass wasn't made by me, it was made by jesse-m and the gems were made by piclet. I made the rest: moving entities, activatables, items and music. If you have any questions on how I did it, I would be glad to share!
About the floaty jump, everyone noticed it feels weird, I was thinking about removing the delay to jump again after you fall and smoothing movements, maybe it is enough for it to "feel" better. I'm very reluctant about changing the mechanic completely (e.g. by speeding up the fall) because it would break many of the level design based on the jump (such as the large gap on level 2). But if it's still not good after removing the delay, I'll consider speeding up the fall (and modifying all levels aswell :'/ ).
The death mechanic is indeed very harsh, as suggested by Here's Andy TV, I'll modify the death system, only reseting to the begining after you lose 3 health.
About the bats: Yes! They need a lot of changes. I thought about making their movement be a sine wave, in order for it to be killed in a more easy way. "not letting them notice the player until they're illuminated by the light" man, this is an AWESOME idea, sounds obvious now, but it simply hasn't come to my mind! I'll sure do this.
Regardings your sugestions...
... about the heart items: Yes! I was thinking about it too, maybe add some secret areas too where you can find hearts.
... about the shield as an upgrade: It sounds awesome, but I don't think it fits the game scope for now, If I finish this game and the release makes at least a little success, I would create a sequel with some new mechanics such as this one.
Again, thank you very much for your feedback and suggestions, means a lot that you took your time to try this tiny game of mine!