I didn't even notice if I lost a heart or not when I went to attack. A sound or red pulse might work. I also had to play at a lower res because jumping crashed the game, but when I played briefly at the higher resolution, there seemed to be some pieces that were high resolution and some that were low resolution. Not everything has to be the same level of detail and noise, but everything should be the same resolution.
Also, in the intro story, the narration says "she thoughts..." as opposed to she thought, and then you guys say thought once again, might be good to change the second thought to ponder or another similar word.
some assets also appear flat while others appear more volumetric, I think you should pick one to achieve a cohesive style.
That all being said, I did feel some nostalgia of playing 2D side-scrollers as a kid, so I think you're hitting your mark tonally.