Musical elements where you wouldn't want them all the time but, when used in the right moment, just really hit the spot.
For me, one of the big ones is a 6/8 time signature. When a track on a predominantly 4/4 album kicks in with a 6/8 and you get that ebb and flow in each bar with snare on the 4th beat, it's just the best thing. 'Olsen Olsen' by Sigur Ros is a stellar example, or Elbow's (love 'em or loathe 'em) 'The Loneliness of a Tower Crane Driver'.