You pose a very good question, as the two types of dresses have become much more similar of late. The fundamental difference, which will never change no matter what the fashion is, is the dress's functionality.
In Standard you never leave closed dance position. Therefore it's important that the bottom of the dress move well, and helpful if the top of the dress has some sort of moving parts as well. Since you don't change positions those parts are not likely to become tangled.
In Smooth on the other hand, one is constantly changing positions which makes moving bits on top both unnecessary and potentially hazardous. Extra fullness and/or length in the skirt is also often desired for those who like to use it while dancing.
Certain elements have come into play because of their function. In Standard, feathers used to be practically required on a dress because of the extra movement and floatiness they impart. Feathers are less practical in Smooth because the drag on the skirt is bothersome when you're doing multiple spins. Slitted skirts have been very popular in Smooth, where they show off kicks (and legs) to great advantage, but they just don't have the same impact in Standard.
The functions the two styles have in common are to catch the eye, flatter one's figure, and perhaps suggest some sort of character...(classy, showy, sexy, lyrical, etc.) This will also never change, I'm sure.
The only other consideration when designing a dress then, is how to be in style, yet stand out and be unique. Aaaaack!
