I think song structure is the most important difference between good psy and bad psy. Everybody has heard that crappy acid stuff where there is nothing but some kind of wobbly synth that hammers away up and down the scale for the entire duration of the song. That is not structure. That is garbage.
Well structured music has a perfect blend of sounds that complement each other throughout the song. Good Baroque music from the Classical Music period is absolutely stuffed to the brim with wonderful strings, twitters, bass, and melodies that are able to complement each other and are pleasant sounding to the ears. Good full-on psy trance has a wide spectrum of sounds that complements each other also, from a rolling bassline, to fat detuned buzzer saws, water droplets, square synths, and melodies. As long as everything blends perfectly, I consider full-on psy to have Classical qualities (without the awesomeness and dancing).
Oh, and this post won't be complete without :drama: