So I wrote some stuff up, thinking about posting it here, and then never did. I think this was shortly after I'd read The Time Traveler's Wife, which some people didn't think of as being terribly SF/F-like (although I thought it was). It was also partly inspired by Zarf's essay on game genres.
So, for me personally, what makes something feel like it's part of the SF/F genre?
1. It is either fictional or a narrative.
This rule differentiates SF from non-fiction that simply discusses science (et al.) in a speculative way. I considered leaving this at just "it is fictional" (as works such as Asimov's "The Endochronic Properties of Resublimated Thiotimoline" can be SF without being narratives), but it seems as though it could be possible to write a true historical account that effectively fit this style of literature. I can't think of any actual examples, though.
2. It introduces some element(s) which are new or different and then explores their ramifications.
This could be something which is new or different for the readers, or it could be something which is only new or different in the setting of the story. The former is much more common, but the latter isn't unheard of. A good example is the Clacks in Terry Pratchett's Discworld series. The notion of high-speed, long-distance communication is commonplace in the real world, but the here the author has looked at how the creation of a network of mechanical semaphore towers would affect his pre-existing fantasy setting.
3. It tries to create a sense of wonder through rational means.
By "rational", I mean that the wondrous-ness is a logical consequence of the elements that have built up to it. It's a "wow, cool" that doesn't just happen because something is cool, but because the author presents all of the pieces, then assembles them into something that's cooler than the sum of its parts.
But that's just me. I'd be curious to hear other people's take on it as well.