I wrote down some notes without a specific order as I played.
Generally, it's a good game. The gravity is lower than you'd expect, which can sometimes help you to control your movement better; sometimes it can also kill you because you don't fall fast enough to avoid danger. The graphics are a bit placeholder-ish (this would usually be the first step to test that all mechanics work before adding "proper", "serious" sprites) and the chosen resolution is too large.
But for a first game, it's pretty good – a fine, enjoyable game without serious problems.
– Control responsiveness
I don't know how you detect being "on the ground", but it's not without flaws. Sometimes it didn't allow me to jump immediately after landing, or from the very edge (a big no-no for platformers), but it's perfectly fine to jump (or rather continue jumping) by sliding under a platform
https://www.newgrounds.com/dump/item/62e49aa39a9e708add22c77e97e814c2
Again, I don't know which method you use, but in platformers, it usually more reliable to detect the ground using raycasts (instead of colliders or something), as demonstrated here:
https://www.youtube.com/watch?v=MbWK8bCAU2w&list=PLFt_AvWsXl0f0hqURlhyIoAabKPgRsqjz&ab_channel=SebastianLague
– Moonlike gravity
Regular gravity would be "faster", but if you prefer it like this, it's okay. It allows you to make longer and more complicated jumps in exchange for the intensity/speed/flow of the game.
Of course, don't change it now, as it would require balancing and redesigning the levels.
– 1920×1080 resolution on the web results in a huge window which doesn't fit on anybody's screen. Add a fullscreen button instead (or simply change the WebGL Template from 'Minimal' to 'Standard' which comes with a built-in fullscreen button). Also, this game consists only from squares and rectangles, so it doesn't need a resolution *that* huge.
https://docs.unity3d.com/ScriptReference/Screen-fullScreen.html
– Is it a good idea to have a character in the same colour as the ground and the platforms? It will probably look better if the protagonist has a unique, different colour than the environment.
– I like the death explosion effect. It's important to have a slight delay after dying to allow the player to comprehend what exactly happened before restarting the level
– Jump button pressed longer = jump higher → great!
Perhaps you could add a different sound for a higher jump to distinguish them. And bouncing on a trampoline doesn't make any noise, but it should since it's technically jumping, too.
– Quit button is useless in a WebGL game. It does nothing and if somebody wants to quit the game, they'll close the tab
– If a trampoline is only one-sided (e.g. in the second trampoline level, where the upper side of the large trampoline doesn't bounce you, only the lower side), it ought to be two-coloured (to let you know which side works as you'd expect)
– You could add medals if you want. A little extra work, but it's worth it because it will lure in more players and make them more likely to finish the game once they start.
https://bitbucket.org/newgrounds/newgrounds.io-for-unity-c/
– Sometimes there's a tiny gap between you and the platform, sometimes you touch seamlessly.
– The difficulty gradually increases and some of the later levels are quite cool
Overall, for a first game, I think it's pretty good.
Well done and good luck with your future projects. :)