There were two main factors that greatly distinguished the two main towers of the World Trade Center from anything that had been built before them: their walls and their elevators.
Prior to the construction of the Twin Towers, skyscrapers were designed to support themselves through large internal columns spaced about 30 feet (9 meters) apart, which interrupted the flow of interior space. For this project however, the engineers came up with a different solution -- the exterior walls themselves would support the bulk of the structure, and they would get a boost from one single column of beams in the center.
This allowed for a much more open plan on every floor of the building, which not only had aesthetic value but had financial worth as well -- the more floor space, the higher the rent the buildings' owners could collect.
Adding to the creation of open floor plans was the design of the elevators. A classic problem in skyscrapers is that as buildings grow taller, the number of residents increases. With more residents, more elevator shafts are needed. But the more elevator shafts there are, the less floor space there is for tenants.
This issue was solved in the construction of the twin towers through the use of express and local elevators. In much the same way the New York City subway system worked, express elevators would take passengers to "sky lobbies" placed on various floors throughout the building where they would then disembark and switch to local elevators to get to their required floor. The use of this system cut the number of required elevator shafts in half, thus preserving valuable floor space.