Well, in short - everything.
A well done D&D implementation up to level 12. Lots of race and class choices. Lots of spells. Plus the many Larian changes.
An extremely rich world full of details to find.
Great storytelling all around. Lots of great characters. Lots of great storyarcs that may or may not happen in your own playthrough.
A welldone combat system. Though I really hate the concept of concentration spells.
Insanely high replayability.
