Josh - I would like to hear more about your thoughts on the construction of waltz and tango in Standard v.s. Smooth
Bear in mind I'm only talking about closed syllabus american smooth, as IMO open smooth is just so completely different that it's hardly the same dance. I'm not experienced at open smooth dancing, but this is my observation.
Regarding waltz, there are two main differences that stand out to me. First is the relative absence of natural swinging movements preceded by a down swing in both bronze and silver. For example, the chasse from promenade in standard blends the down swing from the chasse into the upswing of natural, while in american the chasse is most always followed by closing back up with yet another foot closure and closing to closed position. In silver, there really aren't any real natural swinging movements, as the twinkle from closed to promenade replaces all closed natural figures (in other words, the lady never gets to swing into a natural by going back on her left foot, and vice versa for the man). Contrast that to the naturals in standard that follow the outside change, weave, weave from PP, chasse from PP, outside spin, and maybe others I'm forgetting at the moment. This development of natural swing from a preceding downswing is IMO very important, and you just don't get it in american style waltz. The only thing that precedes a natural turn in bronze american waltz that comes to mind is a closed change. So you never get to blend the continuation from a down swing into a natural.
The other distinction to me is the flexibility in the way the figures are danced and tied together, made possible by the wide array of ways to use the feet other than simply bringing them together (as in american bronze) or passing them (as in american silver). In standard there is the typical foot closure, foot passing on 3 (spin turn, outside change, etc.), whisking actions (regular, back, fallaway, left), heel pulls (man in hesitation), heel turns (impetus for man, corte for man sort of, double reverse and telemarks for ladys), locks (left, right, straight), chasses in closed position, toe spins (man in double reverse), and so on. The majority of the way the feet are used is such that it provides continuity of movement--yet in a way that still maintains the character of what is waltz. Contrast that with american silver, which really has very little to distinguish itself from its american foxtrot cousin in its fundamental basic movement (left and right turns), and little to really call its own.
Tango--syllabus American tango has two main features which IMO distinguish it from standard in its construction, and these are the tango close, and the way that american is usually phrased perfectly, which IMO restricts the expression of the dance, where the highlights and accents can be used at places other than phrase boundaries. The tango close, while a nice figure in itself, is used a lot to phrase out a figure which would otherwise not fit the phrasing. It's nice but it's used in almost every figure, and it tends to make the dance look a bit disconnected, as each figure often looks compartmentalized and distinguishable from one to the next, instead of blending them in more interesting ways. And while they're just two figures, the progressive link and closed promenade really stand out to me as a fundamentally simple and characteristic way to enter promenade position, and close up from promenade position, as compared to how promenade is achieved and closed in american tango. So in a nutshell for tango, it's the timing that is a big differentiation.
On a similar note for tango, I think that slows and quicks tend to be fundamentally misunderstood by many, and especially in american style I see a lot of "stop, stop, go, go, stop" in the basic step. People seem to want to make the dance staccato by stopping, and I would guess it's taught that way 90% of the time, but it is absolutely counterproductive and does the opposite of what it's supposed to achieve. Perhaps in American style the compartmentalized nature of the figures makes this more of an issue, whereas in standard two walks flow right into something else, and thus to me has more continuity (though I would say it's an issue across the board).