In addition to Dev Mode, the Developer's Cut also includes all content from The Dragon Knight Saga, design documents, concept art, and a smattering of behind-the-scenes videos. Oh, and if you already own a previous edition of Divinity II, it's totally free. Most enticingly, it includes a very user-friendly Developer Mode that gives you access to a dev console capable of spawning hundreds of enemies, turning you into any character or monster model in the game, letting you be a dragon anywhere, etc. While I wouldn't call Divinity II: Ego Draconis my favorite fantasy RPG ever (the writing was witty, but the rest of the game was super uneven), Larian's supported it quite well over time, and a new Developer's Cut edition sounds like the cherry atop that dragon-flavored sundae.
Here's a neat and fairly unexpected thing.