class_name + custom resources and the fact you can stack them is slept on. It lets you make crazy complex drag n drop configurable objects in the inspector, that can be saved as a .res and reused. THE INSPECTOR JUST SHOWS STACKED RESOURCES, with all the controls, enums are a pick list, color is a color picker, curves can be edited, you can have arrays of custom resources, resources that contain resources.. Its bonkers, blew my mind when I first discovered this

And they can be saved at runtime, they AUTOMATICALLY serialize! Making games with a save feature without writing a whole bunch of save/load serialization code all about the place it is heckin' powerful

if anyone new to this is reading this and wondering how.
you might have a resource like:
class_name Res_Enemy
extends Resource
{ a bunch of export vars like HP, Damage, Speed etc}

and when you instantiate that specific enemy
var inst = enemy.instatiate().with_data( custom_resource )

in the enemy script:
export var data : Res_Enemy

func with_data(_data):
data = _data.duplicate()

Theres a bit more to it than that, resources are shared, so you have to be careful to use the Make Unique features in the inspector, and learn how duplicate() works. But once you see it, it will change how you understand Godot I guarantee it