Immersion, I like for any door to open up. Even though there aren't any action to complete inside, I hate running into doors that serve better as walls. Unless of course you need to find a key then it's still functioning. Also weather cycles are awesome. I remember I used to play the song of storms whenever it was sunny because the weather helps set the mood for the game. My third biggest desire in open world games is to be able to interact and grab and move items.
80°
50°
40°
60°
Ad 
60°
20°
20°
10°
10°
10°