In games I've played(Thief, Splinter Cell) that are designed for stealth gameplay, lighting is very important. If the player is near bright light sources or in full sunlight, they are easy to detect. Conversely, if the player is hiding in the shadows/darkness, they are much harder to detect. In my experience this type of system works well and encourages you to pay more attention to your surroundings. It could also make the difference between playing during daytime and nighttime(if outside) hours more interesting and meaningful. But there needs to be some kind of HUD indicator for the player to see how the lighting is effecting them. If I remember right, the Thief series used a dynamic lantern icon to indicate how much light the player was receiving.
Another feature that goes along nicely with this kind of stealth system is the ability to extinguish sources of light. In the Thief series, you could shoot magic water arrows at torches to put them out. And in the Splinter Cell series, you could shoot many lights out with a gun. I would also like to see a similar feature in Skyrim... perhaps with some kind of water based spell or enchanted arrows like in Thief?