It isn't entirely fair to say that Campbell "was never much of a writer." In "Who Goes There?" and "Twilight" he wrote stories that are still readable today--and when you compare his work with the competition (some of the best of which can be found in
Asimov's collection
Before the Golden Age), he comes out looking pretty good. Campbell could have continued as a writer indefinitely, but he knew he had an opportunity to do something more--when he decided edit Astounding he said, "as a writer I can write dozens of stories...as an editor I can write hundreds."
Even more than "Man vs. the Universe" stories, he went for "Free, Rational Man vs. the Irrational Masses" stories: see Nightfall, an Asimov classic based on a Campbell idea and edited by him. Through his influence on Robert Heinlein, A.E. Van Vogt, and other writers, he indirectly was a major influence on the libertarian movement. His predilection for stories that appealed to his own adolescent self--an alienated, socially inept, and intelligent boy with an aptitude for science and big ideas--was directly responsible for the creation of SF fandom as we know it today--and, in a less successful way, for the creation of characters like Wesley Crusher.
If you're into geek culture, then Campbell might be the most important person you've never heard of.