To me, the biggest improvement in BG3 is how much looser the gameplay progression is. Since being just two levels behind meant death was all but certain in D:OS2, the path even on an "open" map like the Reaper's Coast was still very much on rails. XP gain was so tight that side quests weren't really optional, even to the point of discouraging roleplay by doing things like passing persuasion checks and then killing everyone anyway to squeeze every last drop out of the map. The first D:OS also really struggled with this until later in the game.

BG3's first large map is a little tight, but even a new player can easily go off script and pick and choose what quests they want to undertake once they hit level 5. Encounters with enemies two levels higher can still be comfortable after that point, even three higher if the player has a good party build or has mastery of the battle system. And the player will want to, because the game is huge. It's such a delight to just go, and it's exciting to see Larian turn a major weakness into a strength.

But essentially, BG3 meets or improves upon every system in D:OS2. The dialogue scenes are the most flashy improvement, supported well by good writing, voice acting, and mocap. The only thing I found to be a step back was the soundtrack. I don't think it's bad, and there are some standout songs for sure, but D:OS2 really excelled in that area both in terms of the quality of the music and how it was used in battle (but then I'm a sucker for cello). It also won't compare favorably to D:OS2 in its current state in terms of polish, but D:OS2 wasn't exactly bug-free on release, either.

A big part of why this game is so big in the zeitgeist right now is because Larian was able to pounce on a lull in the release schedule. I'd call the pre-release hype for this game average at worst for that reason alone. Early reviews were beyond glowing, marking a studio's successful graduation to AAA development with a game that has no aggressive add-ons or DRM. That will spur gaming enthusiasts to generate all the marketing you need.