Stan Lee only wrote the Avengers for a short bit before passing it to Roy Thomas who really expanded upon it. For his run on FF and The Mighty Thor, he had Jack Kirby doing the heavy lifting.
On the whole, I think Lee is a case of being in the right-place, right-time with the right-people, and that brought out the best in him. This is evidenced by the fact that Lee never really had luck creating any new characters after that period outside that. I mean after Marvel, Kirby created the New Gods. Ditko, after Marvel worked at Charlton and created The Question, Captain Atom, The Creeper and he also turned out top quality work for Warren's horror comics.
Lee had the smallest career of the lot and it's especially interesting if you think about the characters that really broke big after Kirby and Ditko left. I am talking of Wolverine, The Punisher, Thanos, Ultron...none of whom were created by Lee.