Good question/topic.
When I think back to my best punts, I think the common theme is that the meet has felt natural and unscripted, and certainly not any reminder that we're "on the clock". If it's a new girl, she's been personable and put me at ease. If it's a regular, she greets me as such and shows (fakes!) happiness to see me again. The transition from clothed to naked to bed is unrushed, and the sex happens in its own time - certainly no Johnnies being tugged over quarter-lobs, or lube being unceremoniously slapped onto bone dry pussies, or "cum for me" after three thrusts.
So, yeah, for me it's definitely the enthusiasm and the passable simulation of a mutually enjoyable tryst.