The core loop, controls, extraction choices and what dying actually costs.
Every class: stats, attack attribute, weapon restrictions and the full skill tree.
Monster stats, elemental weaknesses and resistances, signature abilities and boss mechanics.
Rarity tiers, weapon and armour bases, every random affix and what runes do.
Each biome's boss, hazards and monster pool, plus the elemental reaction table.
Charging, weapon coating, bleed and sunder stacks, stance swapping, terrain chains.
How to spend tree points, which affixes to chase, and how playstyles hold together.
Party loot splits, revives, the red gate, and when other players can hurt you.