A literatura especulativa, vista de fora e vista de dentro...
I'm going to attempt a definition here, or at least an explanation. Speculative fiction writers are apt to treat the subjects of their speculations as if they were real, no matter how outlandish and unlikely: thus, speculative fiction of the highest quality often has a unique reality to it. It employs the tools of mimetic fiction to ground and particularize its flights of fancy, whether they be technological or magical. It takes them literally. It concretizes metaphors. But when mainstream writers venture into speculative fiction, it's all too often a day at the playground, during which they feel free to cast aside the mimetic conventions they normally hold to in regard to plot, character, setting, etc., or a trip to the Olde Curiosity Shoppe, where they can pick and choose among exotic settings, objects, atmospheres, etc., to use as symbols and such in their own stories, which remain highly mimetic in a traditional sense. [...]
Put another way, when mainstream literary writers or readers venture into what they perceive as the realms of speculative fiction, they follow the tedious bromide of the suspension of disbelief. When speculative fiction writers and readers do their thing, they engage the engines of belief.
Paul Witcover, in Locus #586