Intelligence, sense of humor, creativity, and a moderate sense of humility.
The size and type of hat they wear on a sunny day.
That last comment was just silly. My serious answer would be that I am impressed by the kind and/or generous things someone has done for someone else but never broadcast that act. When I was a Christian, I would say this was due to being so focused on Christ and what his purpose was for us versus what the world would say. The principal remains true...do nice things for other people because it is the right thing to do. Besides, it makes most people feel fulfillment when they do them. Just a thought....
Their level of composure in a difficult situation and level of compassion towards complete strangers.
Two of the above plus intellect. The two are kindness and integrity.