The biggest bullet point here is an overhauled armor system, which gets rid of OS2's mildly controversial separate armor and health pools to instead turn armor into a flat damage reducer.
Also, in a rare move for a mod this size, it's compatible with old save files and "bends over backwards to integrate with your favourite class mods, skill expansions and content packs."
I really like the approach this mod has taken to Original Sin 2: It doesn't try to rework the game into a totally different thing, but mainly tries to tone down the most outrageous elements and make many of its systems work better together. It's racked up about 20,000 users and a 5 star rating since December, with frequent updates since then. Divinity Unleashed looks like it's a big hit on the Steam Workshop.