Apologies for the mild necro, but I would like to add my +1 for a day/night cycle that affects gameplay, as well as dynamic wheather (which could also affect gameplay: rain could douse fire; snow could stick to the ground and make it slippery, fog could reduce visual range, etc..)
The original games also had different music themes for day and night, which were really variations on the same pattern/progression. I thought that added a lot to the atmosphere, and it was one of those touches that really made the games feel like AAA titles back when they were released.
"It's the questions we can't answer that teach us the most. If you give a man an answer, all he gains is a little fact. But give him a question and he'll look for his own answers." — Kvothe, The Name Of The Wind