What did Stan Lee do at Marvel Comics that caused them to be so popular?
Stan had a group of creative talent , that he built up around himself, and, if the stories I've heard and read were true, he had given up trying to do things the traditional way. He was ready to quit, so he was free to just do what he liked for what he assumed was his last shot in that world. He had already put twenty years in , starting out as the office boy for Timely comics ( Marvel was one of their publications, just like Detective Comics was one of D.C.'s. The two leading publications became the brands.).
Working with the great Jack Kirby, who was all ready an established artist who was the co creator of Captain America, 9 with joe Simon) he created the Fantastic Four, and had them act more like real people...they had these powers, but were imperfect. They also had hearts of gold, so they weren't completely unlikable. It caught on. he gave an outline to Kirby, Kirby drew the story, adding details and characters ( the Silver Surfer is the most famous example of this, though not the only one). Kirby knew cool, and fantastic, and fashionable, so his work had a cinematic MGM look to it.
Steve Ditko did horror comics, so he was perfect for Spider-Man, who crawls the walls and lives in the dark corners of Manhattan. Ditko was Universal Horror ( or Hammer films, maybe) to Jacks MGM. Stan followed the same creative process- give him an outline, let him draw and develop, and then Stan added the words later.
They had found their method. That was the Marvel way. ( at least at the beginning).