I'd say that while there is a lot of scope for variation -- the speculative writer has to deal with the cliffs he might fall off, while the literary writer, working in a mundane settings, has walls about him to keep him from falling.

Take the "two moons" thing I gave. A speculative writer has to add more clues; he can't just let it stand.

And metaphors of any kind are always more dangerous for speculative fiction, 'cause they could be literal.