Here’s what I mean: in a Star Trek campaign, the problems you face, as a GM, tend to be deus ex machina: that is, the technology is so flexible and powerful that you have to be able to plan for players doing creative things with transporters and force fields and starship weapons that can vaporize whole sections of planetary landscape. Even in a genreal sense, space opera settings share the same sort of pitfalls, such as extremely high-powered personal ranged weapons and the starships that can travel at many multiples the speed of light.

My problem was, I believe, that I couldn’t adapt to a different set of theatric conventions. In other words, my fantasy campaign was more like a sci-fi campaign with a veneer of middle-age and magic. I also didn’t have the toolkit for dealing with the fantasy setting’s equivalents of deus ex machina: specifically, one character was the general of an army, basically, and he seemed intent on taking soldiers with him into every problem.

Granted, like you said above, if you can run D&D, and your players enjoy you GMing, and YOU enjoy GMing D&D, then it sounds like a good enough fit to me. In my personal experience, my group of friends have from time to time asked me to GM — but they don’t ask me to GM something specific. My friends have had experience with how I GM, and thankfully, it seems they enjoy the depth I put into story and character, no matter what the specific thematic window dressing may be.