I've been saying for years that as a scriptwriter, Harlan Ellison is a great short storyteller. The first draft of COTEOF is all the proof I think I need.
For me, the proof is in the later
drafts -- the fact that the producers gave him an unprecedentedly long time to revise and rework his script over and over to fit their production and continuity requirements, and he still
couldn't give them something that could be filmed on a TV budget.
Which isn't a criticism of Ellison's writing. Different formats/media have different requirements, and not many writers can adapt from one to the other. A lot of prose authors can't write scripts, and a lot of screenwriters can't write novels. The parameters are just so different. Screenwriting is more external, more visual, more minimalist, more concise. I like to say that Ellison's imagination was just too big to be constrained by the limits of TV. Which was why it took experienced TV writers like Roddenberry and Fontana to take the core of his story and adapt it into a filmable form.
Although I do think the Roddenberry/Fontana version was a stronger story dramatically in a lot of ways -- we could identify more with McCoy, be more emotionally engaged with him as the source of the threat, than with some guest-star drug dealer. But again, that's a difference in style rather than ability -- Ellison was used to doing standalone, self-contained stories, so didn't think as much in terms of the audience's emotional identification with series regulars.