For me, it's dynamics. When properly executed, it grabs people's attention even when they're not listening faster than any other aspect of music. I've noticed when we first start playing, not to many people are listening, but when we start executing dynamics, it turns heads; a definite way to get people listening to you longer. It's a large part of what keeps a song from being boring because your audience never knows what's going to happen next.
Second to that, or even coupled with it, is a good bass guitar/drum coordination.
Second to that, or even coupled with it, is a good bass guitar/drum coordination.