Most of the changes have been done to either integrate Better Than Wolves into the vanilla progression or to eliminate shortcuts and loopholes in the progression. The description of each change contains both its impact to the game and intention.
Tool recipe changes
Hemp fiber
Stone tools and up require hemp fiber. This has been done to give wood tools more relevance than 3 seconds and encourage players to use all wooden tools until they have enough hemp. It is still possible to do farming and base building so a lot can be done while hemp is growing on the farm.
Moulding
Iron and diamond tools have their handles replaced by moulding. This results in iron tools being gated behind a large portion of Better Than Wolves and prevents players from skipping the iron age by finding iron ingots in dungeons. This also leads to a unique situation where iron ingots are needed for some machines before being used for tools.
Diamond ingots
Diamond tools require diamond ingots which require multiple steps to be produced. Recycling diamond tools has been added as well and they get recycled into ingots. These diamond ingots are used for machines as well.
Gold tool changes (planned)
Some of the gold tools require mystical sticks and mystical hemp fiber. This was done to change up the usual crafting order of tools. Only some of the gold tools need these items to prevent a chicken and egg situation.
Other changes
New Nether portal requirements
Nether portals need their corners to be built from chiseled obsidian. These blocks can only be obtained by crafting various materials from different ages of the mod. This was done to make diamond tools a strict requirement for Nether access to prevent a chaotic progression. One important note is that generated Nether portals do not have the chiseled obsidian corners. This means they have to be upgraded to become reliable portals. This means that generated portals are at a high risk of breaking without a way to fix them unless chiseled obsidian is available.
Ore smelting
Iron and gold ore can not be smelted in the furnace and require a metal forge alongside prior processing using other machines. This has been done to make the smelting process more involved and interesting and to add more depth to the stone age.
Later and more expensive beds
Beds have been gated behind the saw and require fabric to be crafted. This means that beds are a luxury and not available for a significant portion of the game. This has been done to encourage people to a build proper base at night and to plan out the tasks for day and night.