Nothing about it is terrible. The level where you have to get stuck on the spikes to get to the star was a bit annoying because the slime kept sliding and the timing was hard to get right. Nice challenge.
Oh, I would've done everything differently. I tend to overcomplicate things and create strange mechanics that I then have trouble explaining. This is much cleaner and more cohesive than what I might've made. I can come up with stuff I'd add to the game maybe but what it currently has is good and enough and well-executed.
My brain is flooded with loads of new mechanics for this now lol. I guess character responsiveness is one small thing I would improve. Just making the slime have a small reaction when something happens. It seems mostly passive iirc.