I love sims that tell a story, like Wings of Glory and Strike Commander, where a world exists beyond the confines of the cockpit. Missions need more context and purpose than a wall of text presented just before the mission starts. They need a home base that oozes atmosphere, filled with characters and stories that immerse you in the world. You should personally know your wingmen and feel bad when they die. Modern sims lack the human touch, they are all avionics, physics, faceless AI and robotic missions with no heart.