Which factor made it most difficult for soldiers to cross the area between the trenches?

There were numerous factors. The main one was that they could get shot by enemies who would still be hiding in trenches. Another was that there was often things like barbed wires, but also things like landmines. If they ever decided to rush the opposing trenches, they would probably get blown up by landmines placed in front of trenches. Also, WW1 saw the introduction of biological warfare, so many times it would happen that people were sprayed with poisons and toxic chemicals like Yperite at the city of Ypres.