But first, what do I mean by "pillars"? So, I feel like most alternate interpretations of minecraft either change what was added to the game, or just add new features with a beta look. Divergence changes the philosophy that decided what should be added. These pillars are my game design principles for this mod. Every feature in the mod ties back to these pillars in some way. Nothing is just added for the sake of content, everything needs to tie together. With that out of the way, here they are:

Meaningful limitations:

The limitations in this mod don't exist just to make the game harder or frustrating. They are used to alter how you play the game and how you think about it. The main idea is this: You are not here to conquer the world of Minecraft and become a god. You merely exist and live inside it. It doesn't revolve around the player, it exists indifferently.

The world pushes back and that's why there's value in achievement. Sleep is valuable, movement requires judgement, equipment isn't permanent, and progression doesn't make you invincible.

Creativity through constraints:

This one ties heavily into the first pillar. Divergence uses these limitations and constraints to evoke a response from the player. It encourages you to be creative with what you've got. Night's approaching? Beds are not immediately available, so you need to build a shelter or hide. Sprinting is limited? You could build infrastructure for transportation.

When adding a new feature, I always consider how it fits with the others, so that one doesn't make another redundant.(A good example is how glider wings don't make minecarts useless, and how jumping boots/ziplines/boats have their own uses in other situations. They have different uses but aren't interchangeable.)

Another thing is block palette and orientation limitations. By providing you with fewer choices, paradoxically, you are forced to be more creative. This mod doesn't add hundreds upon hundreds of new blocks and items. In fact, it removes a large portion of blocks, and only adds blocks that are actually meaningful or fit the aesthetic, to avoid unnecessary clutter.

A world without final answers:

A really important one: Divergence has no ending or final answers. There's no final Ender Dragon to beat. There's no ultimate goal to achieve. You can explore, go to all the dimensions, get all the items but ultimately it's up to the player what they wanna do in this game.
Minecraft used to have this kind of open ended gameplay, and I think this is what it was always supposed to be: A Sandbox. The game always had progression through thing like tool tiers, but the goal was to make it easier for you to get materials for your builds, and make the world less dangerous.
I think the point of Survival is that it gives building your house or a huge castle value through hardship, and when you finish it, it has a story behind it.
Creative mode is the ultimate sandbox, but in Survival, finishing things gives you a different sense of achievement.

In Divergence, the world is indifferent. It's bigger than the player. Its visuals give it a melancholic and mysterious atmosphere. The world is vibrant, there are animals, plants, but there's no one around. At night there's danger, and you never know what new things might be just over the horizon and beyond the thick fog.
The mod adds new structures to the game, but they are all abandoned and are long forgotten. It makes you wonder. Who built these structures? Were they like us? Where are they now? Divergence doesn't give an explicit answer to either of those questions, making the player wonder what this strange world is.

Meaningful limitations:

Beds & nights:

Because beds in this mod are hard to obtain, and they break after a certain amount of time, they become more of a luxury than an infinite skip button for the night. Nights actually matter again, and this forces you to be aware of the day night cycle, build a shelter, light up your surroundings and prepare for the monsters.
As you progress in your world and light up more areas, build walls etc, your safe space at night will expand, but the nights always come.

Resurrector:

Beds don't set your respawn point, you have to craft a resurrector for that.
A Resurrector is an expensive item which only brings you back once after death. Because of this, in the early game you need to be careful not to die if you venture out far, and even later, death will be costly, and should be avoided.
Resurrectors can be stacked. If one breaks, your respawn point automatically reverts to the previously placed one, so as you progress through the game you need to worry less and less about dying and losing your respawn point, but the threat never truly goes away.

Regen and food:

Food heals your hp directly over time in this mod. Each food has different healing values and stack sizes.
Better foods heal you more and faster but they take up more inventory space, not so great foods take up less inventory space and provide more total HP per stack but they heal you slower. So you have to manage your inventory space based on how you play. If you don't engage in many fights and want more inventory space for blocks then you can choose different foods, whereas if you fight and need to heal quickly, you can choose the best foods.

Movement (stamina, slower movement, new forms of transportation):

Sprinting uses a simple stamina bar in this mod. It is easy to understand and straightforward, but actually you can sprint less at a time than you could with the hunger bar, and sprint jumping uses more stamina.
This means you can't sprint all the time and you'll only use it for short bursts or when you really need to get out of there.
Also sprint jumping will cost you a lot of stamina, so it won't be worth it, on top of that in the mod there isn't much of a speed difference between sprinting and sprint jumping. And the overall sprint speed is reduced compared to vanilla.
What this does is give you a more slower paced, grounded gameplay, where your screen is not constantly bouncing around, and you can just take in the world.

In the mod the elytra doesn't really exist in its vanilla form. Instead there's a Glider Wings item which pretty much acts as a slow falling glider, and you can boost it once.
In the mod it is crucial that one form of transportation doesn't make another obsolete, so there are no horses, packed or blue ice in the mod. Because basic movement is slower there are many options for transportation like ziplines, minecarts, furnace boats etc, and now there's a good reason to build them because they are useful and one isn't better than the other, but you actually need to build the infrastructure for them.

Discouraging large scale automatic farms, and automation:

Big automatic farms are discouraged, because they pretty much go against the whole philosophy of the mod.
With automatic farms you basically build something once and you'll never have to worry about those items, you'll never have to venture out to get them, you can just afk and get all those items, and ultimately you won't feel much sense of achievement from doing so. Automatic farms basically make parts of the gameplay obsolete and provide you with infinite resources. And that's why I don't like them.

Villagers are also not a thing in the mod, for the exact reason that they are too exploitable, and don't fit with the vibe of the mod. In Divergence getting XP and materials is connected to mining, exploring, fighting, and actually being in the world. It is much more grounded and when you actually get the rare items or XP there's more of a sense of achievement to it.

The removal of Mending and the redesign of the repair mechanic

Mending is not a thing in the mod, for the reason that it makes certain items last forever which contradicts the philosophy of the mod.
The repair system has been heavily reworked. Since Mending has been removed from the mod and can no longer be obtained in any form, repair and progression had to be redesigned around that.
The anvil no longer uses the same XP cost for enchanting and repairing. Every tool and armor piece now has a condition tier: Pristine, Worn, Damaged, Fragile, and Ruined. As items are repaired, their condition worsens, which gradually lowers their maximum durability over time. This is meant to prevent items from lasting forever, while still being more forgiving than vanilla repair mechanics.
You can find rare "Repair Books" in the world, they let you restore the integrity of items, though only up to a certain degree. Enchanting and repairing have also been made cheaper in terms of XP, since mob farming has been greatly reduced.

Difficulty settings overhaul and cheating:

This one is important too, if the game gets hard or you die, but there's always a safety net option like turning on cheats, or turning the game to peaceful, then danger and risk don't really exist, do they?
You can always just turn them on and get your items back, or make the monsters disappear. In divergence you can't do that, or rather, it's much harder to do that, adding a lot of friction to doing so.
In the world creation menu there are two options for monsters: Monsters on and Peaceful. During gameplay if you choose monsters on you can cycle through easy/normal/hard, but you can never make the monsters disappear. If you want to play peaceful, there's an option for that, but then it's permanently peaceful.

And then there's the removal of the enable cheats button in the LAN menu. It is a notorious button, which admittedly I used a few times myself too when I got frustrated and lost my stuff or spawned too far away. But by doing that you actually remove risk. Death no longer has value. So that's why that button is removed too.
You can still enable cheats when creating a new world, but you'll need to make that decision, and cannot just change your mind when things get hard.

Creativity through constraints:

Limited block palette and orientation:

In the mod, there's probably a couple hundred modern blocks and items that have been made unobtainable for the sole purpose of creating a highly curated, simple building block palette.
The reason is simple, there's an actual phenomenon called analysis paralysis, which happens when there are too many options to choose from, so your brain cannot actually decide which one to pick. With a simpler block palette and fewer options it is less likely to happen, and you'll be more likely to just start building.
And by having fewer blocks and possibilities, and limitations around what you can build you'll have to be more creative with shapes, colors, block choices, making an ideal environment for creativity.

The Visuals and textures of the mod:

The limited block palette and the older textures also serve to create a more visually appealing art style.
Did you notice that in modern minecraft a simple wooden house looks kinda bad, but in beta things just flow and fit together?
In my opinion, that's mainly because blocks in older versions were more simple looking, less polished, and there were far fewer color choices and different shades to choose from. So the blocks just kinda complemented each other and made a coherent palette.

Giving buildings more intrinsic purpose:

As mentioned, because movement is limited and Shulker Boxes don't exist in the mod, you are encouraged to build infrastructure for transportation.
Nights are a danger so you need shelter. In order to survive you need to build. Many survival limitations and overhauls make you build more, which gives your buildings an actual purpose as a side effect.

New simple painting system:

In the mod the painting system is made to be simple. It only has the 16 vanilla colors, which you can mix on the canvas.
The finished canvas block has a "papery" overlay so it fits in with the other blocks and doesn't stand out as much. The goal was to let you paint pretty much anything while still making it fit in with the world and the blocks.

A world without final answers:

It is really important that there's no final boss and ending in the mod.
Of course in vanilla after killing the ender dragon the game doesn't truly end, you can still do whatever you want after that, but having a final boss completely shifts the progression of the game.
In the back of your mind there's this idea that "there's a final boss, and eventually I should beat it".
So the basic Minecraft progression becomes: Get resources in the overworld, go to the nether, go to a fortress, kill endermen, go to a stronghold, kill the dragon, get an elytra etc.
In Divergence on the other hand you don't need to beat anything. You don't have to go to the void if you don't want to. And there isn't some Op item there either. You can go but only if you want to explore the void, or eventually explore the sky dimension but it's up to you. You could just build castles in the overworld and you wouldn't really miss out on any of the gameplay features.

The overworld:

As mentioned, the overworld is based on Beta 1.7.3, the colors, the atmosphere, the darkness, etc. I feel like visually, that was Minecraft at its best. It's strange but it's interesting, it's not too familiar but still fun, it feels otherworldly.
There are structures in the mod but they are all abandoned. There are no NPCs (except a rare trader guy), the world feels alive but not in a human way. As if nature has reclaimed what was once there. There are remnants but the only fully sentient being is you, the player. It can feel a little melancholic, lonely, or a bit creepy, it is different for everyone, but I think this kind of atmosphere is really where Minecraft was unique back in the day.

The Void:

I've already written about the importance of not having a final boss and that whole section could be reiterated here.
Instead of End there's the Void, and it's not just a rename of the end, it's a different world.
It's still barren, but it has more interesting and strange island generation and regions. The color palette is changed to teal/greenish to create a more otherworldly and distinct feel. You can find Void cities there too but they are much more mysterious than regular End cities.

The Sky World:

The Sky world dimension is the 4th dimension in the mod. There's a nice parallel between the dimensions: The Overworld and the Nether: both are land based dimensions but basically inverses of each other, the same goes for the Sky World and the Void: both are flying island dimensions but one is welcoming and one is desolate and barren.
I think the Sky world is pretty much the most visually interesting dimension of them all . It feels almost like a dreamland. There are also abandoned structures and unique mobs there too for you to explore.