So, I've heard back from the good folks who are assembling the Swords vs. Cthulhu anthology, and my story will not be in it. This is, of course, a tragedy of truly apocalyptic proportions, at least for tonight. I rather imagine I'll have recovered by tomorrow. (I plan to spend the evening sipping on a bit of cinnamon tea with rum whilst shooting things on the Playstation. It's amazingly therapeutic.)
I don't think there was anything wrong with the story per se. The response I received was brief and generic, but I suspect that the issue was just as they said: it wasn't quite what they were looking for. Also, given that they were taking open submissions, I suspect my little story was valiantly battling against a lot of very excellent other entries. So... maybe next time, and meanwhile I have rather nice (if bloody) little short story to either submit elsewhere or build from.
(In fact, I'm wondering if it might become the nucleus of a collection of short stories set in that world, rather than the novel I'd planned to write. This isn't precisely egotism; it's also trust in my proofreaders and their willingness to be honest with me.)