"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.