He had an optimistic vision of the future in which humanity moved beyond its flawed past to explore the stars. Though his personal politics are controversial, his series is an influential part of American culture.
While he created the series, he never had as much control over content as he might have liked. Perhaps in an attempt to gain the influence he sought, he made the rule of "It ain't canon unless I say it is." This has had the unfortunate side affect of sidelining some well written and carefully plotted secondary works. He also rejected STV and ST VI as part of the canon.
This desire to maintain control over his franchise led to difficulties with some of his writers. Harlan Ellison was especially upset by the changes Gene made to his episode "The City on the Edge of Forever."