"Survival Mode as it Should have Been" is a incredibly bold claim, a goal that they aim to reach and a goal they, in many ways achieve.

The number one differentiating of this mod compared to many others is that it rewrites survival, another way to think of it is that its nearly as if the developers use Minecraft as an engine to make a whole nother game. This and some similarities leads to a lot of Vintage Story comparisons. :P.

Gameplay

The mod makes the game have a much stronger survival focus, while Vanilla has you somewhat motivated by survival (building farms for food) its the core of the gameplay now:
Crops take ages to grow - you forage for food.
Food spoils - make vessels and use other methods of preserving food.
Need water - build infrastructure or settle near some water: so on and on and on.

The mod's progression in terms of tools is based around more and more complicated metalworking. Casting copper and bronze (which you alloy) by heating with a pit kiln, all the way until using charcoal forges and (oh dear, very expensive...) blast furnaces to make steel. TFC tool economy is very interesting, stone tools have very short lives, but by the time you get to wrought iron, you realize that these tools have netherite levels of durablity. Forging modifiers replace enchantments, casting gives none and your result of the forging minigame results in tools with what is equivalent to more unbreaking and efficiency to them. It makes the involved mechanics satisfying and to learn and master since you are getting such a good deal out of it!

Large and rock type dependent ore veins make exploration worthwhile, your single deep cave no longer contains everything you need. (sometimes annoyingly, your surface at world spawn may not even contain all you need to get into the copper age)

Worldgen

TFC's world generator is cell based, each containing a continent or ocean in its center, clmate changes as you go east-west (rainfall) and south-north (temperature) (btw it loops, like a triangle wave).

In 1.21.1 it is incredibly impressive, karst biomes at carbonic rock types, glaciers and drumlins at the arctic, mighty stratovolcanoes and shield volcanoes, and various types of coast line. Exploration always feels engaging as the world around you is molded by so many factors. Its hard to explain, just visit the gallery section on the mod page!

Mechanics

The mod has a myriad of mechanics, I don't know how feasible it is to name them, but I will pick a few.

Glassworking, introduced in v3 (for 1.20.1) it provides a engaging (and very laborious, something that really needs to be addressed) process to create jars, glass, and bits and bobs.

Lifecycle and calendar: crops and plants follow a lifecycle, gooseberries may only grow in early autumn, and crops only require the right climatic conditions creating a sense of seasonality in how you grow food. In the newest versions, tree leaves better react to the passage of time, flowering, falling and dis-coloring throughout the year visually showing the passage of time.

Metalworking, the item heating, anvilworking mechanics create value (and some tedium...) in what you make, as metal is scarce and may require reconsidering what you are making and how much. This can be boring and tedious at time and I recommend that players engage with this as far as they feel, no shame in never getting a bloomery and iron, if bronze is working perfectly fine for you.

Integration

The mod integrates itself into and expands on vanilla very well, adding somewhat location based rain, and tilt (which differs by time and latitude) to the sun and moon (yes, afaik eclipses are possible), some vanilla stuff is craftable and usable like armour trims (made through sewing now), tnt/gunpowder, beds (straw beds also let you set spawn point, but you cant sleep on straw beds), and bamboo.

its difficult to integrate with other mods, largely of tedium as so many things need edits but its generally possible.

Extendability

The mod has documentation in game and developer documentation online, its extensive use of data driven mechanics makes it possible and easy to create custom content using its existing mechanics, fostering a large culture of addons. In its code it makes good use of stuff like interfaces and events to allow for addons to make their own stuff like rock and wood types without too much hassle (except, block entities...)

Another cool thing for developers is its use of placed feature tags to make additions to worldgen very easy. (biome modifiers not needed!)

Notes

• Performance is surprisingly good for such a big mod, some places can suffer a bit like chunk generation speed, but comparatively to other mods there are much worse generation speeds than what TFC has.
• Its built-in textures can be a bit rough, a mixture of programmer art and modified modern MC art. Many people prefer to use resource packs such as Vexxed Visuals.
• I alluded to this, but there are a crazy amount of addons for this mod.

Have you ever thought getting a crafting table was too easy? Well this is the mod for you. The worldgen is wonderful and alternative progression is extremely engaging. It turns minecraft into a whole new game. The only thing keeping it away from 5 stars is the performance issues such a detailed worldgen and physics addition can cause. Still my go-to mod when I want challenge.

Terrafirmacraft is the original total overhaul mod, changing every facet of survival gameplay. Past 1.12.2, there is an ingame guide for the player to inform themselves about every change in the mod.

The player is presented with several unusual challenges they must overcome in order to progress; for example, in order to leave the stone age, they must melt metal ore before pouring it into a mold and carefully smith heated metal into useful gear.

The environment is full of obstacles, some passive and others actively harming the player in a task. One of the former is temperature which can be fought by several means, like adapted crops, seasonal preparation and water management. As for the latter, one can think of mining, where the very stone around the player can cave in upon them, crushing any careless miner to mulch if they do not prepare with proper supports.

A large variety of tools is available to the player in order to overcome, and even take advantage of these mechanics. A builder might intentionally cause a cave-in with explosives in order to gather large amounts of cobblestone, or an early player could create elaborate sandwiches to improve their maximum health.

Overall, this mod has a flexible progression arc, and lets the player stray quite far from progression without any punishment. Alone, it can sate a player's appetite for a good challenge, or a relaxing experience for hours.

Unfortunately, the endgame doesn't consist of much- just like any modpack, or the vanilla game, it falls short in progression past the end consisting of the most refined of steels, perfected tool techniques and masterfully cooked meals. At this point, like a vanilla player, the only thing to do is to set your own goals. This is why I rate it 4/5, rather than 5/5.

TerraFirmaCraft, like it advertise itself "Survival mode like it should've been" is a minecraft mod about, guess what: Survival. This mod rebalance and overhaul mostly every aspect of Minecraft by introducing a much more survival-like aspect to the game, explore a completely new world with a revamped worldgen where you now need to deal with cave-ins, food rot, pit-kilns, and semi-realistic tool forging. What you know of vanilla Minecraft won't apply there, there no vanilla furnace or diamond tools and getting simple planks requires you to make a metal saw. While the mod may not be everyone cup of tea due to hard and grindy gameplay, it's a pretty fun mod to play if you ever feel like minecraft is too easy to rush and has no real progression. The mod is a much survival overhaul, but it doesn't have much content past-red steel, in fact, the mod is best played as a early game mod which is where it shines. I would rate 4.5 if I could, but 4 is too low for the amount of interesting content it provides compared to most mods out there.

TerraFirmaCraft is a detailed total conversion survival mod with a lot of custom mechanics, and has some of the best crafting mechanics I've seen to date. The 1.7.10 version in particular is so comprehensive that the mod can pretty much be played on its own. Worldgen, blocks, and items are overhauled, and there is even new music and mobs. Unfortunately, the mid/late game gets repetitive and is mainly just improved methods of metalworking. iirc the mod also had issues with insufficient in-game documentation, which was less of a problem when the 1.7.10 wiki was around, but now that version of the wiki is gone. Still, it's a great mod overall that is worth trying at least once for the experience.