Well, i'd be interested in others' comments on this. When i look at my slow waltz on video i have to admit it looks rather dull, though improving (please say 'yes it is').
I've asked a couple of coaches about this, and thought about it a lot when watching top dancers (why do they look interesting?? etc).
So here are my views:
First, you must have good free-flowing movement with good lowering. This gives good-looking sway and allows upper body action. Secondly, there must be 'light and shade'. I think (

)this refers to the fact that the progression around the floor should be punctuated by moments or two of little movement (e,g telemark or a picture line figure). And nice phrasing helps.