Minecraft wiki entry for End_survival, associated text: If the player falls into the Void, they will die and lose all their items unless they have gamerule keep inventory on.
Endermen spawn infinitely; no matter how many they have killed, there will always be more until players light the area sufficiently.
If players want to survive, they have to bring a lot of stuff with them that would otherwise be inaccessible unless they have an ender chest.
Beds and respawn anchors don't work and they will explode and set fire to the area around the player if that player attempts to sleep in one or set their spawnpoint with one.
Players without either boots with Feather Falling or a potion of slow falling risk falling to their death after shulker attacks.
Nether portals don't work in the End.
The few players visiting the End often travel many kilometers in search of end cities, and will likely take any loot they find there.