Okay first off I am no expert on the intricacies of dance however there are certain things I know to be important to make a dance 'difficult'
Speed and accuracy of execution - If a couple can get a good balance between the speed and the how accurately and cleanly they execute their dance this adds to the difficulty of the dance. For example one of the criticisms the Israelis had and to some extent still have is that they tend to look ragged and frenetic on the ice with moves looking unfinished because they are skating so fast.
Change of dance holds - how difficult the holds and changes of holds are and how cleanly and confidently they are performed. it shouldn't look like an effort or scramble to change hold.
Dance lifts - to be unique and difficult and again how cleanly and confidently they are performed without breaking the flow of the program. The French couple, Olivier and Isabelle, are examples of a couple who perform difficult lifts that sometimes go right and sometimes go wrong.
Footwork - deep edges, match the music, good synchronisation. An example of difficult step sequence would be Albena and Maxims Baroque OD where they crossed each other throughout. It makes it much more difficult to keep the flow and synchronisation.
Other things to consider - is the program an open or closed program. By that I mean does the couple skate apart a lot. Is there a lot of space between them on the step sequences, twizzle sequences etc. Do they skate mostly side by side and facing the same direction. This is a sign of a very open program and an easier program. For a difficult and more 'closed' program the couple should be close together on the footwork sections of the program. They should have plenty of sections where they skate facing each other and be as close to each other as possible, watch their feet and their body line, are they bent at the waist to give the illusion of skating close together when in reality their feet are quite a distance apart.
There is a whole lot more to consider but for me those are the things that I look for first in order to get a feel for how difficult a dance is by comparison to others.