Yeah it's clear that while Roddenberry's original
stories weren't always the best (Return of the Archons, Omega Glory, etc), he was damn good at fixing and tweaking other people's stories to make them work as best as possible on screen. And also with getting the most out of the limited resources they had.
Probably the biggest concern he seemed to have was with keeping the focus of the stories on Kirk and the main cast. It's amazing how many writers had problems doing something even as simple as that!