Something that is always present in a setting--okay, I shouldn't say always, because perhaps some stories are set in space or a vacuum, but anyway--is oftentimes ignored: the floor. The floor is highly important. It not only keeps you from falling to your death (say, in a skyscraper), it might also affect the movement of your character or the possibility of death. Like, if your character is foolishly wearing socks and running down a staircase. This might not be terrible if the stairs are carpeted, but on marble, your character's chance of death via falling down stairs increases approximately 236.7%.
Floors also provide cool sounds--clacking, if heels are moved across hard surfaces. Shuffling, if the floor is outdoors and maybe gravel or weeds. Nothing at all, if the floor is carpet and allows the Bad Guys to sneak up on your oblivious character.
Definitely add floors to your settings--they also help create a nice 3-dimensional space.
By the way, I probably won't be blogging next week--it's GISHWHES!