As Ephraim Katz's Film Encyclopaedia says, Bunuel is one of the few great film makers who isn't interested in "technical virtuosity". (The only other I can think of is Chaplin.) It's something I really like about him. The things going on in front of his camera are fevered enough, it doesn't need for him to be poking the lens up his actors' noses or throwing Germanic shadows across the wall.
His detatchment is positively 18th century. He has a cool appraising eye. If Voltaire could have made movies they would have looked like this.
He doesn't judge, He doesn't laugh, he doesn't nudge the viewer in the ribs. He has absolute confidence in his material.