This was part of a response I got today from an agent who had requested a partial of The Failed Apostle:
[F]rom a market perspective, the SF framework around the low-tech fantasy-like world is a tough sell–you need readers who like both SF and fantasy and the overlap between the genres is surprising small.
I found this to be a surprising statement. Perhaps its because I love a crunchy SF shell around a chewy fantasy center. Perhaps its because many of my favorite books fall into this slot–I’m thinking especially of Gene Wolfe’s Book of the New Sun, and all of LeGuin’s Ekumen novels. I don’t doubt the agent’s judgement of the market–she’s in a much better position to know than I am–but I’m still a little taken aback that this is considered a hard genre to sell.
(It’s also likely that you need to be really good to pull this off, and I suppose that it’s possible I’m not as good as Wolfe or LeGuin yet. I suppose.)
J.S. Bangs 
Well, Wolfe’s Book of the New Sun is among my favourite books as well, but it is also among the most ambitious novels written in English. If that is the standard of excellence you want to measure up against, then good luck, and may your wildest literary dreams come true.
This is probably true, but I wonder if the overlapping segment isn’t growing. I’m thinking primarily of steampunk. It’s not total low tech, but it’s also not shiny spaceships and FTL, either.
Besides, is it really that unusual for people to like both SF and fantasy? I know a lot of people who do.
Perhaps what makes this assessment of the market partly surprising is that cinema has pulled off the fusion of SF and fantasy so shamelessly, if not always effectively, that we can forget that what is easy to pull off in a cinematic idiom is harder to accomplish in print. Obviously it can be done but it’s not as easily done.
I don’t say this as an avid reader of SF, admittedly, in fact I’m revisiting Dashiel Hammett novels at the suggestion of my sister to read some lightweight fiction instead of monographs in biblical literature. 🙂 Having seen more tedious screeds and counterscreeds between Star Trek and Star Wars fans I would say that even in film the gap between those who fancy themselves SF fans (Trek) and those who don’t see what the distinction really ought to be anyway (Wars) does seem entrenched. I think I read more fantasy in my fiction-reading days myself though a lot of academics wouldn’t really want to call Kafka fantasy.
Perhaps it might be useful to make a distinction between “hard” and “soft” SF. Fans of hard SF will have no use at all for fantasy, often on philosophical grounds. Fans of soft SF may have no problem at all shifting back and forth between soft SF and fantasy because the distinctions are not as important. Fans of hard SF want the science proposed in the story to have a provable basis or extrapolation from known science available or considered at least feasible. So in hard-SF FTL is not even up for discussion since it can’t happen, based on what little I know these days about doubts about FTL travel. In soft SF FTL travel is almost a prerequisite in pop culture these days.