I suppose the really important qualities don't normally register with us pax, those of calmness and ability under pressure; if I ever have to evacuate, I know what I'd consider important!
So that brings me to the more easily judged criteria, and a smile certainly helps. As does the offer of the "spare" liquid refreshment when in the back
One of the nicest touches I remember was (in ecomomy on a nearly full BA744 to Phoenix) being addressed by name throughout - I was Gold at the time and the FA had obviously taken time to read the passenger list and remember my name and seat number!
Generally, though, passenger happiness is not necessarily under your control - a late flight and a middle seat is hard to recover from even with the best attention. You have to work with what you've got!
Cheers - SLF