Pi or pmi scales, artificial harmonics, open arpeggios (i.e. when ami aren't plucking adjacent strings), muting strings with finger planting. That's a few examples of techniques that hardly ever occur in flamenco but are common in the classical repertoire.
Bear in mind that some of the techniques may be shared by both but aren't exactly the same. The clearest example of that is tremolo. Whereas flamenco players prefer the quintuplet piami, classical players will often prefer 16-notes (pami for example).
The biggest hurdle for a flamenco guitarist moving to classical is probably the esthetics of tone, which are completely different in each style, and this does have an influence on the techniques you develop. Altering the tone by changing the angle of attack or by moving the right hand closer or further away from the sound-hole is pretty uncommon in flamenco. Or being able to single out a lead tone while playing other simultaneous notes. That hardly ever occurs in flamenco but it is very important in classical guitar (for instance, playing ami at the same time but having one note sound out clearer than the others. Very important for classical. Irrelevant for flamenco).
edit: however, the difference between the two has absolutely nothing to do with flamenco being "dance music" as was suggested in another reply. The fundamental basis of flamenco is cante (song) not baile (dance). Forms like seguiriyas are very flexible in their relationship to the pulse. Not to mention the cantes de levante that are usually played without any real pulse at all.
There is a common misconception today that "in compas" means "cuadrado" (steady beat) but these are too very different concepts. Accompanying dance does usually require more of a steady beat. And perhaps the fact that most flamenco shows outside of Spain are dance-centric has created this misconception that flamenco always needs a steady pulse, needs to be "cuadrado". But nothing could be further from the truth.