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Mining, combat, and pretty much every other interaction with the level and environment are being reworked to make the world feel less like a collection of independent game mechanics and more like a properly polished game.
A few major features you'll immediately notice if you're used to newer versions of minecraft:
No respawning. Save/load only, kind of like traditional video games. Worlds are finite and so are your resources. Liquids are finite. Score is actually important compared to modern MC. You can use tools in your left hand. Blocks can retain visible, directional damage from mining and explosions, Mobs have been overhauled so they're actually formidable foes to face during the night and while you cave. Each mob has its own unique AI and can coordinate like in RTS games.
Your actions and the changes you make to the world can also affect how dangerous survival can become. The world is a place you have to learn and adapt to, and it is not at all a set of predictable systems that can be easily meta-gamed in like 3 minutes
The project is still in active development, so the mechanics, visuals, and balance are still changing...