Minecraft wiki entry for Parity_issue_list, associated text: AI tasks are stored as JSON files.
Behavior packs can be used to change mob spawn rules, biome and terrain features, and loot tables.
Blocks, items, particle, mobs, animations, and biomes can be added using add-ons.
Brewing recipes are data driven.
Entities can be remodeled.
Shared block spaces.
Resource packs use a JSON file called "biomes_client" for coloring water blocks depending on biome, rather than confusing color maps like with foliage.
Said JSON file can also be used to change the fog color of a biome.
RenderDragon rendering engine.
UI interface are data driven with JSON (for old UI) and JavaScript (for new UI, currently in Achievements and Create New Screen UI).
Villager trade tables are stored as JSON files.