The narrative, the branching dialog, the VO performances, the tactical combat, the options for customization. Every game has flaws, even the most 'perfect' ones, but this is about as close to the whole package as anything can get.
Just make sure a CRPG like this is the kind of game you want to play or you'll feel overwhelmed, lol.
This is what playing an RPG is all about. Tons of player agency, not just in story choices, but in how you approach any given situation throughout the entire game.
I've been playing PC games for decades now, at this point. I can easily say, that this is, by and large, the greatest title I've ever played. It stands alone in a tier above S. Do yourself a favor and play the Dark Urge playthrough if you've not already.
Speaking as someone who had zero knowelge of Forgorttan Realms, this is the perfect game to first expereince it.
Will for sure be looking for the rest of the books and games, will be doing many replays