His sense of humor, and his willingness to sass me just a bit every once in a while
The sass makes me realize he was listening and observing, can flip it back on me in a funny, annoying and special way.
Ex: A David Burns book mentions how therapists are conflict-phobic, won't acknowledge a client's anger. Told my T about this and recounted the failed therapist role play. At the end of my session he gave me stupid feedback and I told him that it was stupid. He apologized, said he was looking at the clock. Then he said "You must be angry". Had to say it twice before I got the joke. I stuck out my tongue and gave him the finger I could have left there still annoyed but I left with a smile on my face, happy that we can work together so well.