The contributions of Gene Coon are almost as significant as those of Gene Roddenberry. Coon gave us Khan, the concept of the Federation, and the Klingons, but then, he also gave us "Spock's Brain," so I guess it all evens out. Actually, I hadn't looked at the infamous episode in a long time prior to writing this, so I looked at some highlights (lowlights?) on YouTube. You could get a great drinking game going by taking a shot every time someone says "Spock's brain!" I've heard it said that the premise isn't bad, only the execution. Maybe that's true.
The Klingons were created during a time when the Cold War was still running hot; thus, they were commonly looked upon as the Soviet Union to the Federation's United States. When we first see them in "Errand of Mercy," though, it's in relation to the mysterious, enigmatic - and therefore more interesting - Organians. By the time a Season 3 episode like "Day of the Dove" (written by Jerome Bixby) rolls around, the Klingons have become more established as aggressive antagonists.
It would take the movies, particularly The Search For Spock, and the launch of TNG, to provide the makeover that would truly cement their popularity. I still think it's unfortunate that Enterprise solved the forehead/non-forehead mystery; speculating about it was more fun! (Fun fact: the name comes from a former LAPD coworker of Roddenberry's named Lt. Wilbur Clingan!)
Coon died in 1973 at age 49, so he never lived to see Trek become the international and multicultural media franchise it became. A tragedy, but enough books and articles have been written about TOS that fans know his name and what he did, which is good. Someone as important as Coon shouldn't be forgotten.