I know it's a subgenre of scifi and fantasy, but what are the defining characteristics? What makes a story slipstream?
Slipstream fiction may or may not have significant fantastical or science fictional elements. It seems the most critical factor in labelling a tale as slipstream is the sense of cognitive dissonance created by a realistic setting with elements of the sf&f genre introduced to produce a surreal effect. See Slipstream(genre) and this review.