Trust, willingness and eagerness to explore each others bodies to find what makes them tick, what turns them on (do more of that) and what turns the off (avoid that). Paying attention to how their body reacts to your touch is key!
I love to watch my husband's abdomen and thighs as I give him a blow job or a hand job. When I am doing something that feels totally amazing I know it, not because he is groaning, rather his abdomen (right above his public hair) will quake. His body sends me clues telling me he is getting close to ejaculating, if his thighs get antsy along with his abdomen.
What makes my husband an excellent lover is he truly knows my body better than I know myself. For instance, we recently got the Pure Wand and used it tonight for the second time. Everyone said in the reviews that there was a learning curve to using this toy. Well, he knew exactly what needed to be done and had me soaking the sheets in a matter of minutes. I need him to give me lessons on usage.