His unpredictibility: after so much time together I still can't predict his actions,there's always the surprise-element.
His ability to communicate(and of course,his ability to understand when to shut up
);He attaches a lot of importance to the communication in our relationship,and of course makes time for it,and I think this was the key of the success. His ability to have a conversation with different types of persons and still feel good about the conversation and make the other feel good(!). I can't stop smiling when I see him kneeled on floor,talking to a child,letting him hear his own heartbeats with the stethoscope and saying things like"what do you hear,colleague?"...
His self confidence(and sometimes his arrogance :p)