I think the vastness of it stands out to me. It has a rich lore and history, and, instead of spoon feeding that history to you through data logs or journal entries, you actually live it and experience it. Hell, one storyline actually takes you into the past to experience first hand the great war that shaped the present day. And that storyline came out like 8 years after the game was released.

On top of that there are many side stories about the locals that are just as engrossing as the major plotlines. And you are free to pick and choose what you experience and when you experience it. While the game stays fairly serious most of the time, there are plenty of comedic relief moments and just flat out silly side stories to experience.

Add all this onto the fact that you are generally experiencing this with a group of people is just the icing on the cake. I still remember the day we beat Alexander. I didn't jump for joy because we just beat some random superboss, I did it because we finally came to the conclusion of an epic storyline that we were all anxious to finish.