I love how you can create such expressive faces with so few characteristics. Severus, with only one eyebrow, and no nose(I mean, Severus--Severus with no nose?!?!), manages to convey such disgust and impatience. Harry is just endearing.
On another note, I really like the way they are highlighted by their black robes against the gray brick walls. It really gives a sense of depth. Also their luminous eyes are lovely!