You know what I love? Simulated physics. I love watching objects fall and tip and fling and crash and crumple in dynamic and believable ways. Simply by implementing a believable (not necessarily realistic) system of physics and gravity that affects the objects in the game word, a game is given a literal and figurative weight, and is opened up to all kinds of dynamic and exciting outcomes.
Half-Life 2 is nine-years-old. Nine years! Since then, simulated physics have become so common-place as to hardly be noticed.