People rightly praise Stan Lee and Jack Kirby who created so many Marvel titles, but the true genius of Marvel Comics in the 1960's was in spreading the Marvel Madness to other creators. And boy did they get hot! Roy Thomas and John Buscema are like comics' supernovae here.
Many of the epics of today's Marvel comics are just re-interpretations of these classic stories by Roy Thomas and John Buscema (with an assist from Werner Roth and Don Heck).
It is easiest just to list the innovations:
The first appearance of Ultron, the robot nemesis who will be featured in the upcoming Avengers movie.
The Black Panther moves over from Fantastic Four, setting the stage for him to take the center stage as a hero in the Marvel Universe.
The Black Knight returns as a hero and disappears before the Avengers can induct him into their ranks.
The dead hero/villain Wonder Man's brother the Grim Reaper challenges the team for the first time.
The time traveling Scarlet Centurion (who will one day become Kang the Conqueror) stops by and forces the team to fight an earlier incarnation of itself.
In the art department, John Buscema experiments with layout and design in a way even Jack Kirby didn't. Costume design is clean and powerful. He pays attention to how fabric moves. His buildings add character to a scene instead of just being a backdrop.
Roy Thomas even threw in the very first X-Men vs. the Avengers.
Well worth a read.
Roy Thomas and John Buscema are just getting started here.
The best is yet to come.