To me, as a dancer, good tango arrangement has to include these five elements:
A variety of rhythm: single-, double- and occasional quadruple-time beats, as well as silent, implied beats.
Smooth, flowing melody.
Surges in the melody (that “swinging” feel).
A variety in the arrangement, where different instruments or voice interweave and come to forefront of the orchestra.
As a dancer I hear and express the qualities 1-4 to in the dance. Bandoneon, with its ability to produce in a skillful hands either bellowing, or sharp accented, or quick keyboard sound (all three can be overlaid), is particularly suited to express these qualities. Here is good introductory article on Bandoneon technique.
Tango vals and milonga share the same five qualities. In vals they are overlaid on top of vals signature "1-2-3-pause" phrase. Some turns fit this signature perfectly, hence turns are often used in vals. However vals need not to be limited to turns. Here is an example of perfectly musical vals dancing by Alberto Dassieu with few turns.
Milonga has more incesant drive than tango (silent beats are de-emphasized). That's why "electrons-plus-drum" adaptations of milongas (Milonga Sentimental, Baile a Beneficio) by Otros Aires manage to stay fairly close to the original feel. Classic tangos by the same band (La Yumba, Niebla del Riachuelo), of course, are necessarily bastartized.
By the way, if you you are interested in the electronic tango, Otros Aires is a good choice - by far better than the mechanical compositions of Gotan Project or the plain dumb arrangements by Bajofondo Club.