Imo what this game did the best isn't in any way revolutionary. It's mostly about polishing things that universally are good in rpgs but might be not easily accessible for average gamer. I am talking about things like:
- character building - you have variety of classes that are distinguishable from eachother. Take for example any 'hardcore' crpgs like 2e infinity engine games or even pathfinder km or wotr. There are so many classes, some of them extremely similiar, that you can really get confused if you don't want to spend hours on reading descriptions and other fuckload of texts. Bg3 classes are very easy to distinguish but they still retain a level of complexity by having for each of them three different specialization and other flavors
- likable companions - I know that it is controversial and paradoxically I don't like a single character, but because they are very humane and have a modicum of complexity,
they make gaymers invested in their stories. I am pretty sure that reddit crowd was more willing to finish this game simply to uncover what sh hides about her past or to fuck a bear
- interactive world - probably one of the most important factors. The amount of different approaches you can have during combat and all kind of silly things you can do outside of it is so entertaining, that people are willing to play this game simply for the sake of finding out anything new about this. It less on rpg spectrum and more on, well, being a videogame. This is really a simple ingredient for a good game - having lots and lots of reactivity.
So these imo are qualities that make this game good. It's a case of combination of good elements, nothing especially original