Is it their immediate proximity to the putting surface ?
Their depth ?
The angle of the face of the bunker ?
A slight elevation on the green side that diverts water but also kicks the ball forward ?
Their collecting nature ?
Their relationship to the putting surface when viewed from the prefered and non-prefered angles of attack ?
Their visibility or invisibility ?
Their size ?
What U.S. courses embody most or all of the above design features ?