> Everything, just be honest
Controls feel pretty good. Except the noticable "delay/freeze" when you grab a crystal. When this was a deliberate decision, I would encourage to overthink it. A seamless VFX instead would give a much better rewarding feedback.
Game could need some more variety or puzzle aspects. Something that let's me want to keep on going. Reading your devlog, I realize I haven't seen any dialogs or Lore in game - maybe I just haven't noticed them.
> The Art isn't perfect ...
... yes, but your simplified design is a good start. I would suggest having a look at Will You Snail or this one. As NeatGame was already mentioning: a lot can be done by "just" adding some post processing effects.