I took notes as I was playing. First I want to say great job on the graphics and the animation! You have huge potential to be a great pixel artist and make beautiful games if you keep practicing!
Ok now everything else will be constructive criticism. So here we go. I hope this helps you because that's why I took my valuable time to write it! :)
No controls explanation - I tried all keyboard controls and can only figure out left/right/jump.
There's this weird motion blur / vibration when walking that's annoying.
Signs are all blank with an exclamation mark - why are they there then? I tried every single keyboard button and couldn't figure out how to read the signs.
Why does it says idle/walk? This is like a developer / debugging feature that should be removed.
Platform with waterfall looks like I should be able to jump on it but I can't - please differentiate clearly between background and foreground.
Solid brown blocks look like background but I can stand on them.
Jump mechanics - I'm always hitting my head with too much vertical velocity - can't easily reach places because of it. It's way too much unnecessary jump power.
No way to avoid first set of spikes because of the issue with the jump mechanics. The level design (tunnel height) does not match the jump intensity so it feels buggy like I'm being punished for having too powerful of a jump.
It's hard to get coins due to the insane vertical jump power.
I lost after walking off an edge with no understanding of why I lost - it didn't look like I died, I didn't fall or anything, the game just suddenly ended. Why didn't I fall first?
Looks like there's a secret platform behind me when I first start but I'm not rewarded for jumping on it - so why is it there?
There's this platform within like 1 pixel short of being able to jump on it, such that it looks like a mistake not intentional - give unreachable platforms a good gap so it's obvious.
There's no coin tracker - why should I get coins then?
I'm standing on this this brown square with a green squiggly border, and due to the camera I can't see anywhere safe to jump. Always show the player where to go next.
I successfully made all the blind "leap of faith" jumps and I just reached a dead end. I don't know where to go, but since the game didn't make it clear, I'm not going to die 100 times trying to figure it out any more.
There doesn't seem to be any way to win the game or have multiple levels - this is not a finished game, but a prototype. Please finish it before asking people to play it or explain that it's just a prototype for testing _________ (fill in the blank) feature (such as testing collecting coins or jumping). Research / know the difference between a completed game and a prototype. Nothing wrong with a prototype, just know the difference!
Some other things that would be nice to add: Sound effects, music, humor, tutorial, some story, a boss or some surprise or reward at the end and a clear game over message.
Thanks for having the courage to share. Keep working on this project and it can become truly great!