The visual quality. Older flight sims seem to have Nintendo64 game graphics compared to Flight Sim 20, that´s why all third party developers had to seriously ramp up their cockpit quality, cockpit realism, and the overall graphics quality also on the outside of the airplane.
And of course the complex systems in airliners. That Fenix with all the electric systems and the hydraulics and pneumatics - pheeeew that´s some intense coding and hundreds of pages of program code to bring realism that far, and bring her to life!