Full backward engine versions compatibility thru compatibility translation layers that only the Epic Games team could write to ensure that any asset and any code could be quickly migrated without any issues from older versions to the newest one.
APEX Destruction for meshes on mobile devices
An official text scripting language for the visual BluePrints (so that can be coded faster having both a visual and a textual version without using C++)
Complete and updated documents on every single API and both BP and C++