just read the GAME_DESIGN_PRINCIPLES. some minor problems.
First "Beds shouldn't set spawn": yes, i agree its OP. But there are some reasons to be able to set respawnable points. some people like being able to move away from their spawn just because they dont like how it looks around there. Or its a server and people want separate spawn points at their own houses.

Second "Trade-offs": yes, equipment should have debuffs, but pls dont make it like "double your jump but increases fall damage" dont make the equipment punishing for what it is intended. especially when you dont need to add that as a downside since theyll already jump higher and therefore take more fall damage. and a quick one "A golden pickaxe is strong, but uses rare resources and is slow to swing." this one looks familier. mc golden pickaxe is about as fast as diamond but rare ressourse and weak. No one goes for the golden pickaxe, they rush for diamonds.

Third "No Unbreakable Tools": I can see where this comes from, but i fear that this might encourage playstyles where people will be afraid to use their rare ressourses for mining, and then spend more time doing boring stuff with their weaker and slower tools even though they have better stuff. There is a reason minecraft players decide to grind and AFK just for mending. even though its the most mind numbing and boring activity possible

Fourth "No Fast Travel": to put it bluntly, This will only be possible with either 1 of 3 things happening, either distances beetwen ressources are small Or the methods of making infrastructure are cheap and easy to set up. imagine travelling half an hour down a cave just to realise you forgot wood for torches. while you might argue this is to punish the player for bad planning, which would be true. But is an unnecessary amount of friction at the same time. The fix. terrarias Potion of Return. Allow them to travel back to their spawn, and create a temporary one time use wormhole

Honestly I really love the idea of no unbreakable tools. I like that there's always a reason to get more resources unlike Minecraft and Terraria where you just get everything once and feierabend. As for mining being tedious with lower tier resources, I think Cubyz already does a good job at balancing ease of obtaining with ease of use. Take copper for example, sure it may not be the strongest material but it's speed and ease of obtaining makes it quite pleasant to use.

This is a reply to @Fyrstikkeske

"No Fast Travel"

This has been changed to "No Teleportation," as it should've been.

"Beds shouldn't set spawn": yes, i agree its OP. But there are some reasons to be able to set respawnable points. some people like being able to move away from their spawn just because they don't like how it looks around there. Or its a server and people want separate spawn points at their own houses.

This one will need more discussion before it's finalized, but in my opinion, respawn points you can just set anywhere is overpowered and has resulted in a lot of meta-game shenanigans in Minecraft. What I changed it to is having Spawn Points be seen as more "Check Points" that you come across in certain structures or create yourself (at the cost of rare materials.)

Alternatively, you could allow the spawn-point block to be beds, but have them drop nothing when broken, to prevent abuse of moving spawn-points. This option is less ideal.

"No Unbreakable Tools"

I've since added an explanation as to why this is a thing. I am not the best at wording however, so I hope it's not too confusing.

I think "Cubyz is a sandbox" could be added somewhere near the top.
The player should be able to build in peace, and choose how they would like to play.

I do want to add one more point that I forgot before this is merged.

Looks good to me.