A list of things mod developers will need to change and some of my personal favorite things. Full list of changes here - https://minecraft.gamepedia.com/1.13

Nice
				
			- The Flattening changes a bunch of block states and values - https://minecraft.gamepedia.com/1.13/Flattening
 - The World Generation algorithm has been redone.
 - The entire command system has been completely overhauled - https://minecraft.gamepedia.com/Commands
 - Data packs replace hard-coded values- Loot Tables, Recipes, etc. - https://minecraft.gamepedia.com/Data_pack
 - Water has been reworked. Its no longer a block, water and lava flowing blocks have been removed. Waterlogged is a tag which states whether the block has water in it or not. Items float to the surface.
 - The block ID limit is gone. 

 - Some blocks collision box now match their bounding box. cough Hoppers cough
 - Ooh, chests can be placed right next to each other.
 - Undead mobs now sink
 
Nice
			
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