Pretty much everything.
The story grabbed me from the start and never let go. It's very compelling throughout and the ending was very dramatic (even though the catch is that we pretty much know overall how it would end).
I greatly enjoyed the gameplay which was a mix of not-straightforward exploration, lightsaber dodge/block/swing combat, and a few other types thrown in from time to time. I think that all of the gameplay was borrowed from other games a bit here and a bit there, but for me that's not a negative.
I also greatly enjoyed the characters, dialog, and acting.