Hello and welcome to Feedback Quest 9! My name is Hythrain, and I'm one of the hosts for this lovely event! I'm also one of the streamers, so both the game and this feedback are being done live! If you want to see the see the VOD, let me know and I'll link you to it when it's uploaded to YouTube!
Right off the bat, as a game in early development, it's honestly pretty solid in the design. I liked how easy it was to understand and I had a lot of fun with the puzzles. So awesome job there. That said, there is absolutely a ton of room for improvement.
First, I recommend not using ESC to just quit the game, even in a playtest. Instead, ESC should bring up a menu where the player has the option to return to the title screen or to exit the game. I say this because my muscle memory when playing games during these events is to use ESC to access a menu to pause the game, tab out of the game to write down notes, and then go back into the game and hit ESC to unpause. This made me close the game several times, to the point where I stopped playing after I did it once despite still having 20 minutes or so that I could dedicate to the game. It's just super frustrating to accidentally close the game and have all my progress on a level reset.
Speaking of menus, there should be a way for the player to change mouse sensitivity. This game's sensitivity is really high and I know a lot of people would like to reduce it.
I would also suggest changing how saving works. First, make the game autosave when you reach a new level. Then make it that when the player saves, they save their progress in a level. That way, if a player needs to stop playing mid level (or in my case, closes the game by accident) they're able to retain the progress they made.
During the tutorial level, I noticed I couldn't begin moving until the level text went away. Personally, I'd say to just let the player begin controlling immediately.
Next, I would suggest making it that the player doesn't need to keep holding down the E button in order to hold objects. This can make it difficult to also move around, since the finger most people use for pressing E is also the finger they use for pressing D. If, for some reason, you like it being this way, then at least add an option to switch between Hold and Toggle so people who dislike it can switch.
I also eventually learned that using the left mouse click can ALSO be used to hold blocks. This is just superior to holding E and I don't understand why this wasn't told to players. Maybe it wasn't intended, but I do like it more. If you plan to get rid of using left mouse click, then do my suggestion above. If you plan to keep it, drop using E and instead just use the left mouse click.
In Level 2, you have platforms that the player has to use to cross a gap. I recommend adding a brief stop to these (and to any other moving platforms that you'll expect the player to use) as doing so will allow players to be able to get on them without feeling rushed. It doesn't need to be a long pause, just a second or two.
In a similar vein, it occurred to me during Level 2 how much the boxes obscure one's view (which in turn made jumping onto moving platforms even harder). What I'd recommend is that when a player is holding an object, that object becomes transparent so they can see through it and alleviate this problem.
I also generally found that when I'm holding boxes, they're able to clip through the floor while being held. While you have code in there to ensure they don't get lost (I did experiment with dropping boxes into holes), it still kinda made me nervous. If you're able to, maybe try to work in some special hit detection where the box can't pass through floors, walls or other objects while being held. This may not be needed depending on how extensive your safety nets are.
One thing that I felt iffy about was how the world looks. Everything is black and white, and it can sometimes be challenging to discern what things are. Some colour would be an idea, especially for the lines that you see with the glasses in order to help determine what each line is in relation to.
The next few notes are just little things, so I'll just bullet point them.\
1. There's a weird lip on the exit door for Level 0.
2. I noticed a weird line above the targeting reticle.
3. The pointer part of the arrow is off centre from the arrow line.
Lastly, I have an idea that may really benefit this game. I'd recommend trying to create platforms that can be jumped up through from below but which you can't drop through. Think of the clouds in Super Mario Bros. 3. You could then set these up above wind turbines that shoot the player up, making it that the player begins on a platform instead of needing to maneuver to one.
That's all I got. Overall, this is pretty solid so far and I can see it being way more solid it a lot of the ideas above are implemented. Hope to see more from you in future events!