Day and night cycle are already in and the world has fully dynamic lighting thanks to the CryEngine.
We are going to be working on NPCs a lot. I think player NPC interaction is very important in order to make your game immersive.
This! For me personally, atmosphere and the illusion of a living, breathing world is very important.
Gothic is a prime example of this, especially the city of Khorinis in Gothic 2. People going about their business during the day, peddlers selling their wares, smiths smithing and such, and then retiring to the tavern at dusk, having a drink, and finally going to bed later.
That entire game just oozed atmosphere… I still remember wandering in that dark forest with all the wolves and a sunny day suddenly turning dark and oppressive walking under the green canopy, with howls in the distance…
Good times.
Edit: And I'll chime in with the rest, and say gameplay, story and (rewarding) exploration takes precedence over graphics for me as well.