-constant atmosphere with sounds - wind, rumbling, baby crying, rats scurrying
-illogical passageways, they open up where you started. One more is its a never ending passage in one direction, but you can exit if you switch directions.
-signs and markings from a previous adventurer (similar to the messages left in Portal)
-mobs of small, fast, flying swarms of easy to kill creeps (think pigeon-rat hybrids)
-partial loss of control, there are times when you can only aim and shoot, or you can only run and the weapon shoots by itself
-loading screen is pit and the pendulum setup - you are on a bed in the middle, the walls are closing in and a pendulum with a sword attached to the front is swinging and lowering towards you